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.)
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
- 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.
- 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.
- 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
- bundt pan or ring mold
- offset spatula
- food coloring
- piping bags
- 260 g all purpose flour
- 100 g cake flour
- 226 g butter, unsalted and softened 2 sticks
- 75 g coconut oil
- 440 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
- 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.