This dreamy fluffy white cake, made out of a batter which will make you nostalgic for your favorite childhood birthday cakes, is baked in a bundt pan and to be drizzled with your favorite ganache or glaze. By itself it is quite a treat. But, cut it open and boom! A gorgeous little rainbow centers every slice, absolutely delighting everyone around you (and your own inner child!). Perfect for any occasion when unicorns and/or rainbows are called upon for their power to make us smile.
Omgosh omgosh omgosh.
I had the idea for this cake wayyy back in… November? December? IDK but sometime late last year when everyone else was making Christmas cookies and predictably my brain was jumping ahead to Spring and sunshine and leprechauns. Back then, it wasn’t the time so I wrote it in my idea list and left it there. I patiently waited for you all to get over the pie and the gingerbread, and then the many citruses (if I see one more blood orange post on my IG feed I will poke my eyes out) and then all the chocolate and hearts in February (#guilty), and then back to the blood oranges because apparently they are going out of season and what an absolutely dreadful thing that must be! (sorry, I just really hate blood oranges!).
But guys, today it’s March and it’s time. Today we talk about this cake. My idea was to use a bundt pan and somehow get a rainbow inside with some of the batter so that when you cut it open, OMGOSH There’s a rainbow! It’s magic!!! WHAT! Yes, I’m five but this is what happens to your brain when you become a mama to a lil girl who is obsessed with unicorns and rainbows and awakens the inner child in you.
For this recipe, I had the yummiest white cake recipe to work with but my technique needed fine-tuning. I went web-hunting and found this article which showed a similar technique as what I had in mind but the result wasn’t quite what I was after. With 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 different pan 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 do the same! So my friends, you’ll use this recipe and my notes to bake a cake that will delight every rainbow-loving person and make kids squeal with delight! Happy rainbow baking!
Hidden Rainbow Cake
You’ll need to use a large bundt pan to make this, any shape will do but consider how the rainbow will look once the cake is baked. A bundt is usually turned upside down so we’ll build the rainbow beginning with red - making the red fatter and gradually decreasing the width of the border of the circle as you move through the colors, ending with purple. 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. (Bonus: my ring mold also made it looked like a giant donut!)
My recipe is a mash-up of various white and lemon cakes I use regularly. I use a bit of cake flour here because of all the stirring that goes into coloring the rainbow batter. I chose a ‘white’ batter because I wanted the canvas for the rainbow to be pure so the colors would stand out. To do this, you need to leave out the yolks from your recipe. So, 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 in removing the yellows. You could use all your yellows and leave out the coconut oil - this would make your cake less ‘white’ and more yellow which would in turn affect the colors of the rainbow slightly.
I used 5 colors to make my rainbow as I found that the orange wasn’t necessary and too many blues/purples just created a deeper coloring on the bottom. Build the rainbow you want but make sure you use five colors!
I did a simple white vanilla glaze on one of the cakes (1 cup powdered sugar, 1 teaspoon imitation vanilla and 1 tablespoon milk) and a white chocolate ganache (100g white chocolate, 60g heavy cream) on another, but really the options are limitless! Color it to whatever tickles your fancy. My daughter suggested a dark chocolate ganache and I was so tempted - if you try this I need to see it!
Update June 2019: a how-to video I posted on my Instagram highlights.
260g all purpose flour
100g cake flour
2 sticks butter, 1 cup
⅓ cup coconut oil
2 cups sugar
1 teaspoon salt
2 teaspoons pure vanilla
1 teaspoon imitation vanilla
2 teaspoons baking powder
4 egg whites, at room temperature
1 cup buttermilk, room temperature
Assorted food coloring
First, make your batter:
Preheat oven to 325.
Butter and flour a bundt pan.
Begin by sifting together the AP flour and cake flour into a bowl. Set aside.
Cream butter, coconut oil and sugar together for a full 5 minutes or even more, until very light and fluffy. Add salt, vanillas, and baking powder and whip for a few more minutes.
With the mixer on, add egg whites, one at a time, allowing each to be fully incorporated before adding another. This beats air into the batter so add slowly.
Add half of your flour mixture, and mix until just combined.
With mixer on low, add buttermilk in a slow drizzle. Add rest of flour and beat just to combine.
Build your cake:
In 5 separate bowls, add about 4-5 tablespoons of batter. Using gel food coloring add red, yellow, green, blue and purple to each bowl and stir to combine the colors. Use about 2/3rds of the remaining batter to fill your bundt pan. Leave the last third for the top, to cover the rainbow.
You can use a spoon to make your rainbow but I found a piping bag kept things pretty tidy.
Begin by filling your bag with red and make a circle within the bundt on top of the white batter. Make the border of the circle pretty thick by circling several times - you are going to be making 4 more circles with thinner borders within it. Use the back of a teaspoon to smooth out the red batter and even it out in preparation for the yellow.
Do the same with the yellow, making it slightly less thick (leaving a slight space on either side so the red is still visible. Continue with the green and so on, until you have a thin circle of purple.
Use a spoon to gently add the rest of the white batter on top. Smooth top over with an offset spatula.
Bake for about 45-50, checking for doneness after 40 minutes.
Allow cake to cool for about 15 minutes in pan, then invert onto a cooling rack.
Once cake is totally cool drizzle with ganache or glaze.
Lucky Charms Cereal Milk Version:
a few hours before making the batter, allow the buttermilk to sit at room temperature in a bowl with about 1 cup of the cereal from the lucky charms (not the marshmallows - these will color the batter!). When it’s time to use your buttermilk, strain it through a mesh sieve to remove the cereal. Press the cereal down to extract as much flavor as possible.
you will also need 5 colors to make this rainbow (even though the lucky charms version only has three! but too few colors won’t give you a cake rainbow - believe me, I tried!). I expanded the blue-yellow-pink marshmallow by adding green and purple in their appropriate places. But I can imagine a traditional one that starts with pink would be just as pretty.