Filed under: Bundt Cakes
March 1, 2019

Hidden Rainbow Cake

Rainbow bundt cake: a dreamy fluffy white cake with a nostalgic flavor of ‘birthday’. Slice it open and Oooh! A perfect rainbow centers every slice. Perfect for any occasion when unicorns and/or rainbows are called upon for their power to make us smile.

post featured image

A dreamy fluffy white cake with a nostalgic flavor of ‘birthday’ baked into a bundt. Slice it open and Oooh! A perfect rainbow centers every slice. Perfect for any occasion when unicorns and/or rainbows are called upon for their power to make us smile.

OMG! Hi!!! I’m so excited to share this cake with you today!

I had the idea for this wayyy back in late November, back when everyone was making Christmas cookies and my brain was jumping ahead to Spring and sunshine. It wasn’t the time to make it then, so I wrote it in my idea list and patiently waited for us to get over the pie and the gingerbread, and then the many citruses and later all the chocolate and hearts in February…

Surprise! It’s a Rainbow Cake

But, now it’s March 2019 and it’s time. Today we talk about this cake. The idea was to make bundt cake and when you cut it open, OMGOSH There’s a perfect rainbow! Magic!!! (Yes, I sound like I’m five but this is what happens to your brain when you have a two year old lil girl who is obsessed with unicorns and rainbows, lol.)

 

surprise rainbow cake

 

 

I already had the yummiest white cake recipe to work with but needed to figure out how to execute it to get the perfect rainbow arch. This cake had a similar look & technique but the result wasn’t quite what I was after. On my first try I got the teeniest rainbow in my slices. On my second, I got a giant, bright and beautiful rainbow but it was too close to the edge of the cake to give me a centered, U shape. On my third try, I used a ring mold and figured out how to better divide the batter and! Got a perfectly centered and gorgeous rainbow and now I can tell you how to make it too =)

 

 

Rainbow cake in a Bundt

To make this, you need a large bundt pan and piping bags. Most bundt shapes will work but consider how the rainbow will look once the cake is baked. A bundt is usually turned upside down so we’ll need to build our rainbow beginning with red. I tried this in a brilliance bundt, a tube pan and a ring mold – the last gave me the best results because of the curvature on the bottom which gave me a nice upside down U shape that made the perfect rainbow. The pan I linked to takes you to a 9” ring mold, this is the one I have and use. It can be a tight fit for the batter and will seem to rise above it but I’ve never had problems with the cake overflowing.

Why this surprise rainbow bundt recipe will give you the perfect rainbow

  1. The pan: Use a ring mold or a bundt pan that is curved on the bottom (I’ve had readers use the classic nordic bundt with successful results) – this is what leads to the perfect U arch. 
  2. Dividing the batter: a heaped 1/3 cup goes into each color of the rainbow, most of the white on the bottom and just a bit to cover the top. This I figured out through trial and error and by noticing how cake batter rises and slides inward while baking. 
  3. Five colors: Any you want – we’ve done them all here (including a lucky charms one – scroll below)! But for some reason when I tried six this threw off the ratio of batter I’d carefully balanced so let’s stick with five. 

Why this white cake recipe is the best for a surprise rainbow cake

My recipe is a mash-up of various white and lemon cakes I use regularly. I use some cake flour here because of all the stirring that goes into coloring the rainbow batter. We need ‘white’ batter because so the colors can really stand out. To do this, you need to leave out the yolks from your recipe but this can dry out a white cake. Based on a tip from BraveTart who does this in her white mountain cake recipe, we’ll add coconut oil to replace the moisture lost. The coconut oil does not add any flavor (make sure you buy refined to avoid the coconut taste). 

The cake itself is a cross between a pound cake and a white cake; it’s dense with a perfect nostalgic crumb but it’s also utterly soft and so sweet with vanilla. With or without a rainbow, I adore this white cake recipe. 

 

How to make a hidden rainbow cake in a bundt pan

Let’s start with getting the perfect white cake consistency: butter, coconut oil and sugar will be creamed together. I always add my flavorings (salt & vanilla here) to the butter/sugar stage because the flavor is enhanced when added with the fat. Leavening goes in this stage too. 

Then cream the mixture, five minutes should be enough but get in there with a rubber spatula and scrape down so that it’s all being mixed together well. 

The egg whites go in one at a time, let each incorporate before you add another. And do the scrape down as needed. 

Lastly, the flour and the buttermilk (sometimes I use kefir). Sift in the flour, this leads to a nice fine crumb in the cake especially since cake flour is almost always lumpy. My. mixer likes to make dry ingredients fly out so I usually turn the batter with the rubber spatula a bit before it, covering it with a cloth also helps!

Buttermilk, hopefully at room temperature but it’s not a must will go in slowly as the flour is being mixed in. When you are done you should have a very light, creamy (maybe slightly bitty looking) cake batter. Don’t forget to scrape down at this stage too.

Now the fun part!!! Get five bowls and scoop 1/3 cup batter in each bowl. Then color them – make sure the colors are nice and bright – I often add two drops then mix and see how much more I need to add.

P.S. sorry about the extra bowls. Making magic often makes dirty dishes.

Now get all the colors into their individual piping bags. Smaller ones are better for this. If you don’t have piping bags use sandwich bags.

Scoop in most of the remaining white batter (leave about 1/2 cup for the last step) and smooth it out. Use the back of a spoon to make a space for a ‘moat’, this is where the rainbow will be. Pipe the red first (or whichever color will be your top since the bundt will be turned upside down) in a ring shape as shown.

Then smooth it out as wide as you can but keeping a border on its inner and outer sides. The back of a teaspoon works wonders here. The second color goes in next, but when you smooth this one down make sure some red is visible on either side ie. make the colored rings more narrow. Keep going doing the same thing with the remaining colors.

You might be asking yourself, why did I use piping bags for this instead of a spoon?

You could use a spoon but piping bags give you more control over where the color is going so that the resulting rainbow will be similar all around.

Once you get to the purple you should still be able to see the other colors on either side of it. Carefully slide the pan on the counter to kind of ‘settle’ the batter.

Then put the remaining batter on top but very carefully – don’t disrupt the rainbow you just painstakingly created. Use the back of an offset spatula to get the white in an even layer (doing your best to get some of the top white to touch the white that’s under the rainbow.

Now take a deep breath – that was hard! It’s time to go into the oven.

If you used the cake pan I link to here, the batter will seem like it’s going to spill out of the cake. If you measured everything by weight this won’t happen (no promises if you measured the sugar or flour by cup and used too much!).

When it comes out of the oven it might sink a bit, but it’s fine. Let it cool in the pan for a bit then turn it over onto a cooling rack.

This is the hard part – wait for it to completely cool before slicing it! You’ll get better cuts this way.

Surprise Rainbow Bundt cake toppings

I did a white chocolate ganache (100g white chocolate, 60g heavy cream) for one of the cakes, a raspberry glaze for another (1 cup powdered sugar, 1-2 tablespoons raspberry puree). The options are limitless. Start with 1 cup powdered sugar and add 1 tablespoon juice your choice getting it to glue-like consistency (elmer’s).

Questions you might have about Sam’s Rainbow Bundt Cake 

Would a different pan work? 

Layering the colors in will always get you some rainbow inside, it’s the arch you might not get if you are using a different pan. But many readers have made this in different bundt pans and even without the upside down U it’s still a lovely surprise. 

Can I substitute the coconut oil? 

If you use refined coconut oil you won’t taste it in the cake. In fact please don’t use unrefined – save that stuff for your hair! I don’t have a perfect substitute for it (after all, it’s standing in for the egg yolks which we left out to preserve a perfect white cake batter). 

Can I make my own buttermilk? 

A lot of recipes tell you a mix of milk and vinegar but this isn’t a good sub as those mixes are often more acidic and less fatty (creamy). I use kefir in a lot of my recipes where buttermilk is called for or I’d suggest thinning out some greek yogurt or sour cream with water (60% sour cream, 40% water).

Can I reduce the sugar? 

You but probably not by much, the cake won’t taste the same and will likely be drier. 

What kind of food coloring do you recommend? Can I use natural colors? 

Natural colors don’t always bake up the way they look when you first mix them in. Often they change completely; reds and pinks become orange, some become an unattractive brown. I use wilton or americolor gel coloring. 

 

Hidden Rainbow Bundt Cake Recipe

Surprise Rainbow Bundt Cake

Hidden rainbow bundt cake! In a perfect white cake sits a magical surprise.
surprise rainbow bundt cake
5 from 10 votes

Ingredients

  • 260 g all purpose flour
  • 100 g cake flour
  • 226 g butter, unsalted and softened 2 sticks
  • 75 g coconut oil
  • 420 g fine, granulated sugar 2 cups
  • 1 tsp fine sea salt
  • 2 tsps pure vanilla extract
  • 1 tsps imitation vanilla
  • 2 tsps baking powder
  • 4 large egg whites
  • 250 g buttermilk 1 cup

Method

  • Preheat oven to 350 F. Butter and flour a large bundt pan.
  • Begin by sifting together the all purpose flour and cake flour into a bowl. Set aside.
  • Cream butter, coconut oil, sugar, salt, vanillas and baking powder for a full 5 minutes until very light and fluffy.
  • With the mixer on, add egg whites, one at a time allowing each to be fully incorporated before adding another.
  • Add the sifted flour mixture then turn the mixer on low and slowly drizzle in the buttermilk.
  • In 5 separate bowls, add 1/3 cup of batter. Using gel food coloring add red, yellow, green, blue and purple to each bowl and stir to combine the colors. Use most of the remaining batter (more than 2/3 of it) to fill your bundt pan. Leave the last bit for the top, to cover the rainbow.
  • Place each of the colored batters in a piping bag (you'll have five) and snip of the top. Use the back of a teaspoon to create space for the rainbow as shown in the photos above.
  • Pipe a red circle into the moat then use the back of a spoon to smooth it out so that it's as wide as possible while still leaving a white border on the inner and outer rim.
  • Now add the yellow, leaving a red border on either side. Each following color will decrease in width - see photos above. Spoon the rest of the white batter on top with an offset spatula, careful not to smoosh the rainbow.
  • Bake for 45 minutes, checking for doneness at 40. The bundt is done when you can touch it and it springs back.
  • Allow cake to cool in pan for about 15 minutes, then invert onto a cooling rack.

Video

Notes

June 2019: a how-to video I posted on my Instagram highlights.

Did you make this recipe?

Share & tag me on instagram @buttermilkbysam

Rate + Review

What do you think of this recipe?

Your email address will not be published.

Recipe Rating




  1. 5 stars
    First of all, I have to say that I’ve made SO many white cakes and this is the BEST recipe I’ve ever used. It will be my new go-to.
    Second, I was out of buttermilk, so I tried your suggestion to use sour cream, and the cake still turned out wonderfully!
    And finally, your directions on how to make the rainbow were easy to follow and understand, so I credit you for being able to pull it off my first time! Thank you for sharing your beautiful (and delicious) art with us!

  2. 5 stars
    I made this and it was amazing- the taste was incredible, and the colors were beautiful and vibrant, and a lovely surprise.

    I made a few substitutions, and as a result my “rainbow” didn’t really form – the colors barely moved and it looked more like a heart, which was just as satisfying. That being said, I’m hoping you could help me understand what might have caused this?

    Here are the subs I made:
    – I did make my own buttermilk despite your recommendation.
    – I only had 60g coconut oil, so I used that and added 15g canola.
    – used all vanilla extract instead of some imitation.
    – used a NordicWare bundt pan instead of a ring mold.

    Thanks!

    • Hi Lisa, so glad you liked it! The shape of the rainbow is really pan dependent, any pan that is curved on the bottom, when piped as shown, should lead to an arch in the rainbow. The other subs probably didn’t make a huge difference -although they may have lead to a thinner batter which possibly could affect the shape, but I think the real culprit here is the pan.

  3. I’ve been fascinated with this cake since the first time I saw it. It’s so pretty and elegant.
    I made it twice already and it’s a cake loved by all!
    The recipe and steps are super clear and easy to follow. I love the touch of coconut, it makes a difference in yumminess.
    My icing didn’t work out the first time and I thin it was my fault so I tried again without and still it’s a stunning cake. Thank you Sam for sharing your great ideas with us 🙂

  4. 5 stars
    I made this in a standard bundt pan with pretty good success. Next time will need to fill the bottom less and make more of a moat. Cake was made for a birthday and the birthday girl loved it.
    I didn’t have imitation vanilla, so just used my homemade vanilla paste. The batter was scrumptious.

  5. 5 stars
    Just amazing!!! I’ve made this 4 times now and shared it with friends, family, and neighbors. Everyone loves it. Because the main feature is the rainbow, I wondered if the taste would be underwhelming—I was so wrong. It is a beautiful, tender, delicious cake. The coconut oil is genius, bringing that perfect moisture that the best cakes have . My rainbows have been a bit different each time, and I love that no rainbow is perfect, but they all come out adorable. This is great to give to people going through a hard time. It’s childlike, simple delights that get us through the darkest times, and this sure is a delight. 💜

  6. 5 stars
    What a wonderful cake! I need to be a bit more aggressive with my moat making, but I’m quite happy with the color, the rainbow whale surprise inside, and the flavor of this cake! I used Lorann Oils Princess cake emulsion in place of the artificial vanilla extract and it worked quite well! Thank you for the recipe!

  7. Hello!! I just got your recipe and it looks super delicious, I’m excited to make it next weekend. But I have a doubt!! How many grams of coconut oil should I use to replace each egg yolk?

    • Hi Andrea, thanks for the note. I updated the page recently with a recipe card and must’ve forgotten to add it. The coconut oil is in the list of ingredients. Hope you get a gorgeous rainbow!

  8. Hi! I’d love to make this cake for my daughter’s bday. I have a question. Can i use 2 full eggs instead of 4 egg whites? If so, do i need to adjust the fat content and by how much?
    Thank you in advance!

  9. 5 stars
    This recipe was so fun to make this St. Patrick’s Day! The cake is so light and tender from the egg whites/cake flour, and I was pleasantly surprised with how well the hidden rainbow turned out!

  10. 5 stars
    This was my first time making a bundt cake and it came out totally amazing! I could only find a 6 cup bundt pan so I just halved the recipe and it was perfect. The detailed directions and IG video were super helpful. What a pretty and delicious cake to celebrate the first day of spring!!

  11. 5 stars
    This cake is magical and delicious!!! Using both vanillas is key, and the directions are spot on perfect! I wasn’t sure it was going to turn out, but following the exact directions created a perfect inside rainbow Bundt

  12. 5 stars
    I loved making this cake! The tutorial found on your Instagram page was exactly what I needed. Super easy to follow and a lot less difficult than I thought it would be. Highly recommend making the Lucky Charms version…I made it two days ago and there’s nothing left 🙂

  13. 5 stars
    Delicious and fun, I made it for inauguration because rainbows were my mood that day. The instagram story was very helpful – even using a brilliance bundt (only pan I have with a hole) pattern got a decent curve because I followed the hint to use more batter. Question for future cake color combos / to understand what you mean by balance: are 5 colors necessary because fewer colors would blend together too much, or is it because you need 5 rings of batter?

    • Hi Dee! So glad you guys loved it and inauguration day sounds like the perfect occasion for this ! About the five colors, every time I used less than 5 colors I didn’t get the same perfect rainbow (I tried it a few times with the lucky charms version). It could’ve been that I wasn’t using enough colored batter or it could be something else but I’m just not sure!

  14. Hi
    Can I make this vegan ?
    Using aquafaba as a replacer for egg White
    If not vegan then just eggless ?
    Do you mean 1 cup melted butter ? What would that be in grams ?
    What do you mean by imitation vanilla?

    • Hi, nope sorry this cannot be made vegan. I suggest finding a different white cake recipe that is intentionally eggless or vegan. The butter is not melted, it should be softened. 1 cup is 8 oz. Imitation vanilla is a specific type of vanilla, different than ‘pure’.

  15. I am so excited to make this but can’t find the video. Is there another link you can send? Thank you!!!

  16. Hello! I really want to give this a try. I ran across it on Pinterest and it looks lovely! I was wondering if you had any photos of the piping over the rainbow process. I’m super visual and a picture would help reassure me that I’m doing this right!

  17. Why does the recipe call for a mix of real and imitation vanillas? Going to try my hand at this in a couple of hours! 🙂

    • Hi Stefanie! I love this question! I think the combination makes the best flavor for the cake. I think the pure vanilla is necessary to give the cake good vanilla flavor but the imitation (when used with, not as a solo) gives it a hint of that childhood nostalgia taste, like the funfetti boxed mixes that were so popular in the 80s and 90s. Hope the cake turns out great!