When I showed M some ideas I had for her birthday cake, she squealed her approval. Usually she wants some variation of cannoli but this year we wanted to be considerate of her slumber party guests. I also really wanted to make something really colorful and fun because M is such an enthusiastic artist. For years I have really wanted to make a rainbow layer cake and this year it seemed just perfect for M’s birthday. When the birthday girl and her friends gasped (they actually gasped) and exclaimed “this is sooooo cooooooool!” I knew I had a winner.
In the name of full disclosure, for years I have entertained this cake as a fun way to call attention to Pride Month or National Coming Out Day and then didn’t. Some years I felt like cake was kind of insignificant compared to the struggle for equality. Other years I didn’t have a crowd to share it with and as much as I like cake I know my limits. A lot of what held me back was the fear that this would look like something that belongs on Cake Wrecks and really be a mockery. So the idea kind of sat toward the bottom of the list and I’m so glad that M’s birthday inspired me to make this cake at last and to share it in the wake of the Supreme Court’s decision granting marriage equality to everyone.
Also in the name of full disclosure this cake looks a lot more difficult than it really is. It’s time-consuming but it isn’t terribly difficult, if that makes sense. I used the same vanilla cake base I used for the funfetti cupcakes (and the Oreo cupcakes) and made a double batch of my new favorite buttercream frosting. To add some more fun, color, and sugar to the cake, I decided to coat the outside edge with sprinkles. That last step is completely optional but it did help me save some time because I didn’t have to worry too much about smoothing out the frosting or adding any other decoration.
Rainbow Layer Cake
for the cake:
- 3 1/3 C. all-purpose flour
- 1 tsp. baking powder
- 1/2 tsp. baking soda
- 1 tsp. salt
- 2 C. sugar
- 1 C. butter, melted
- 4 egg whites
- 1/2 C. Greek yogurt (vanilla or plain) or sour cream
- 1 1/2 C. milk (almond, soy or cow’s)
- 4 tsp. vanilla extract
- Gel food coloring in red, orange, yellow, green, teal, and violet
for the frosting:
- 2 C. salted butter, room temperature
- 2 Tbsp. vanilla extract
- 8 C. confectioner’s sugar
- 8-10 Tbsp. heavy cream
- optional: 1-2 C. sprinkles (or “jimmies,” not nonpariels)
Preheat oven to 350 degrees.
Grease and flour the bottom and sides of as many 8-inch round cake pans as you have (I used 3 at a time).
In a medium bowl mix together the flour, baking powder, baking soda, and salt and set aside.
Melt the butter in a microwave-safe bowl and pour it into the bowl of a stand mixer fitted with the whisk attachment.
Whisk the sugar into the melted butter on low speed; the mixture will be gritty.
Whisk in the egg whites, yogurt, milk, and vanilla extract on low speed until combined.
Slowly mix the flour mixture into the wet ingredients using the paddle attachment until no lumps remain; batter will be thick.
Divide the batter evenly into 6 small bowls.
Stir in the food coloring, one color per bowl, until the desired color is reached. I used Wilton gel food coloring and the colors in the bowls were pretty true to the color of the baked cake layer.
Bake the cakes for 8-10 minutes. Check them at 8 minutes; the cake is done when a toothpick inserted in the center comes out clean. Baking for some colors took as long as 20 minutes but check them at 8 and keep checking them every few minutes.
Let the cakes cool in the pan for about 5 minutes, then turn them out onto wire cooling racks to cool completely.
Repeat the baking and cooling process until you have 6 differently colored layers of cake.
When all layers are completely cooled, prepare the buttercream frosting.
Beat the butter in the bowl of a stand mixer fitted with the paddle attachment until the butter is smooth, about 3 minutes on medium speed.
Add the confectioner’s sugar one cup at a time. Start mixing the sugar in on the lowest speed until it is incorporated, then add the next cup. Once you have added 6 cups of confectioner’s sugar, beat the mixture on medium-high speed until the frosting comes together. If needed, beat in the remaining confectioner’s sugar. It is VERY likely you will need all 8 cups of confectioner’s sugar.
Add 8 Tbsp. of the heavy cream and the vanilla and beat on medium-high speed until the frosting comes together. If needed add the remaining heavy cream one tablespoon at a time. The frosting should be fluffy but stiff enough to support the cake layers.
To assemble the cake place the violet layer on the bottom and spread an even layer of frosting over the top, all the way to the edges. I used about 1/3-1/2 cup in between each layer.
Place the teal/ indigo/ blue layer on top of the frosted violet layer and repeat the frosting process. Repeat the layering and frosting process (after the blue layer is green, then yellow, then orange, then red) until all of the layers have been stacked.
Frost the top and sides of the cake and decorate as desired.
To coat the sides in sprinkles, gently press handfuls of sprinkles against the sides of the frosted cake. Gently tilt the cake plate to remove the excess sprinkles.