Rainbow Layer Cake Recipe 
Yield: Serves 16
Prep Time: 45 minutes
Cook Time: 15 minutes
Cooling Time: 2 hours
Total Time: 3 hours
Ingredients:
For the Cake:
3 3/4 cup (450 grams) all-purpose flour
1/2 cup (56 grams) cornstarch
2 1/2 cups (495 grams) granulated sugar
3/4 teaspoon salt
1 1/2 tablespoons baking powder
1 teaspoon baking soda
6 large egg whites, room temperature
2/3 cup (132 grams) vegetable oil
1 2/3 cups (378 grams) buttermilk, room temperature
18 tablespoons (254 grams) unsalted butter, room temperature
2 teaspoons vanilla extract
1 teaspoon almond extract (optional)
6 colors of food coloring
For the Frosting:
2 cups (454 grams) unsalted butter, room temperature
11 cups (1248 grams) confectioners’ sugar
3/4 cup (170 grams) heavy cream
2 teaspoons vanilla extract
3/4 teaspoon salt
Sprinkles for decorating, optional
Directions:
Preheat the oven to 325°F. Line 8-inch pans with parchment paper and grease with a light spray of non-stick cooking spray.
Make the cake batter. Mix the flour, cornstarch, baking soda, baking powder, sugar, and salt in the bowl of a stand mixer or a large mixing bowl. Cut the butter into 1-inch pieces and slowly add to the dry mix while the mixer is running on low speed. Continue to mix until the flour mixture looks sandy and crumbly with no large pieces of butter, about 30 seconds.
Pour in the oil and buttermilk, mixing on low speed until thoroughly combined, 1 to 2 minutes. Slowly add in the egg whites, one at a time, followed by the vanilla extract and almond extract. Mix until just fully incorporated.
Color the batter. Divide the batter into six bowls, approximately 330 grams of batter per bowl. Color each with the desired food coloring, starting with a few drops, mixing, and adding more if necessary.
Bake the cake. Pour the colored batter into the prepared cake pan and bake for 15 minutes or until the top springs back when lightly pressed and the edges just start to pull away from the cake pan. Let the cakes cool for 10 minutes in the pan before turning out onto a wire rack to cool completely. Wash the cake pans and repeat as necessary until all layers have been baked.
Make the frosting. While the cakes cool, prepare the buttercream frosting. Using a mixer, beat the butter on medium speed until light and creamy, about 2 minutes. Add confectioners’ sugar, one cup at a time, continuing to mix. Pour in the heavy cream, vanilla extract, and salt. Beat together for 30 seconds, then increase to medium-high speed and beat for an additional 2 to 3 minutes, until light and fluffy.
Assemble the cake. Starting with the purple layer and working up to red, place a layer of cake down, cover with frosting, add another layer and more frosting, until all six layers have been stacked. Coat the outside and top of the cake with a thin crumb coat of frosting, then refrigerate for 30 minutes to set the crumb coat. Use the remaining frosting to spread along the outside and top of the cake, and decorate with sprinkles.
Notes:
Use gel food coloring for the boldest colors in the rainbow cake layers. I use AmeriColor gels for this cake.
To keep cake layers perfectly flat, use Bake Even Strips.
All components of this cake can be made ahead of time. The cake layers can be baked and frozen for up to 3 months. The buttercream frosting can be made up to two days ahead of time. The entire cake can be assembled a day or two before serving.
To prevent the cut cake from getting stale, wrap leftover slices in plastic wrap or push wax paper against the exposed edge to minimize contact with air.
(Here’s the link to the recipe, so you can see the picture! It’s awesome, I just can’t upload it for some reason.)
Recipe courtesy of Bunsen Burner Bakery.com