Uncharacteristically, I didn’t have a clear idea for my daughter’s third birthday cake. We talked about unicorns, shades of purple, and rainbows but I couldn’t land on THE perfect idea like I did last year when I made her this melting ice cream cake. Life with her and the now two month old baby is pretty chaotic and mental space is hard to come by. I made a very small, basic white cake for her party earlier that week which, tbh, was more for candle blowing than eating. So on the day of her birthday, I had to scramble for ‘the’ cake. This came about in the space of less than 24 hours: the night before, I asked her what she wanted, she responded: “rainbow, inside and outside, with stripes outside and no ruffles.” Alright then!
Truth is, if she had her way this blog would be exclusive to rainbow and unicorn themed bakes. Whenever I tell her I will be baking, she suggests (in the adamantly insisting way toddlers do) that I make this hidden rainbow cake. So it should have come as no surprise to me that she wanted a rainbow.
The morning of her birthday, we awoke to no cake flour, so we threw hoodies on top of our pjs, slipped sock-free feet into sneakers, and hustled to the store at 7am. As soon as we got back I made my batter, split it into seven bowls, colored and baked them. We took her out for waffles for breakfast and then for some fun outings. When we got back, I made my buttercream, then assembled and decorated the cake during her nap.
She awoke to this cake. Her face lit up when she saw it and she commented it was ‘so light’ (which I think was a good thing?). Earlier that week we were treated to a full rainbow in our DC skies (first one I’ve ever seen!) which she absolutely loved, and the colors of this cake came pretty close to what we had seen. I was so pleased because I find food coloring incredibly frustrating; colors are always brighter or stronger than I intend for and they never seem to match up to the sticker color on the bottle. But this, well, it looked like a true rainbow! Perhaps it was the luck of not overthinking any part of the cake, and just throwing it all together at the last minute.
true color rainbow cake
I used KAF’s tender white cake recipe for the batter. It produces a very soft, light cake. Any white cake recipe that will yield enough for two 8” cake pans will do.
You’ll bake each layer in a 6” cake pan. I have three of these 6” pans so this went by quickly for me, but if you only have one or two it will take longer. Remember to wash, re-flour and re-butter every pan before pouring in a new batter.
You’ll need to split the batter equally into seven parts. To get perfectly even layers you can weigh the batter and divide it by 7, then weigh each bowl of batter individually. If you eyeball them (as I did) you can use a cake leveler after the cakes are done to equalize them, although this is a bit less reliable as cake height will depend on how much batter you put in.
When using food coloring in buttercream, it’s best to make it the day of as colors often fade after a day or two. Also, keep your cake out of sunlight as this can cause them to fade as well.
- 326g cake flour
- 333g granulated sugar
- 12 TB (170g) unsalted butter
- 1 tablespoon baking powder
- 3/4 teaspoon salt
- 4 egg whites plus 1 egg, at room temperature
- 1 cup whole milk
- 1 tablespoon pure vanilla extract
- 4 sticks salted butter, softened
- 6 cups organic powdered sugar
- 2 teaspoons imitation vanilla
- 1 tablespoon pure vanilla extract
Preheat your oven to 325. Butter and flour 6” round cake pans.
In the bowl of a stand-mixer fitted with the spatula, add all the dry ingredients and butter. Mix on low until the mixture is fully combined and like sand.
Add egg whites and egg one at a time, beating until fully incorporated and scraping the bottom as needed before adding the next.
Add vanilla and milk, mix until combined, scraping the bottom as needed.
Split your batter evenly into seven bowls and color with one drop of color per bowl. Stir until color is smooth. Pour into prepared cake pan(s) and bake each layer for 15 minutes.
Allow cake layers to cool in pan for 5 minutes, then turn onto a cooling rack to completely cool.
To make buttercream: beat butter on high for 5 minutes, until very light and fluffy. Add vanillas and beat to combine. Sift in sugar and beat until thick and creamy.
To assemble the cake: place your first cake layer on your turntable or plate and smooth over it a few tablespoons of buttercream. Repeat until all layers are on the cake. Frost your cake fully with a thin layer of buttercream. Use a bench scraper to smooth the sides and top of the cake (this is your crumb coat). Place cake in the fridge while you ready the next step.
Split the remaining buttercream in to seven bowls, add one drop of food coloring to each and mix well. Place each buttercream color in a piping bag so you have seven piping bags. Cut off about half an inch from the top of the bag.
Pipe buttercream in circles around cake, in the order which you want your rainbow to appear. Once all colors are piped, use a bench scraper to smooth the buttercream while turning the cake. This page does a wonderful job demonstrating how to do the buttercream with photos.