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Geometric Rainbow Cake

birthday vanilla flavored cake made in the tradition of the Malaysian Sarawak Layer Cake (Kek Lapis Sarawak): one sheet cake baked in increments to create a large layered cake. The cake is sliced and then strategically stacked to create a gorgeous geometric design.
Course Dessert
Cuisine American, Malaysian
Prep Time 1 hour
Cook Time 45 minutes
Servings 1 very tall loaf cake

Equipment

  • 13 x 9 " cake sheet pan
  • pullman loaf pan

Ingredients

Double Vanilla Cake

  • 390g or 3 cups cake flour
  • 170g or 1 1/2 sticks unsalted butter at room temperature
  • 113g or 1/2 cup coconut oil
  • 300g or 1 1/2 cups granulated sugar
  • 3/4 teaspoon baking soda
  • 1 3/4 teaspoons baking powder
  • 1/2 teaspoon fine sea salt
  • 6 large egg whites
  • 360g or 1 1/2 cups buttermilk at room temperature
  • 2 teaspoons pure vanilla extract
  • 1 teaspoon imitation vanilla

Double Vanilla Buttercream

  • 2 sticks unsalted butter softened
  • 1 teaspoon fine sea salt
  • 2 teaspoons pure vanilla extract or 1 tablespoon vanilla paste
  • 1 teaspoon imitation vanilla
  • 3 cups organic powdered sugar sifted
  • 2-3 tablespoons milk or cream

Instructions

  • Make your buttercream first: beat the butter with the salt on high until light and fluffy. Add the powdered sugar and beat to combine. Add the milk and stir the buttercream. Set it aside in an airtight container.
  • Prepare a sheet pan (13 x 9”) with parchment paper (this will make it easier to release later).
  • Make a note of the weight of your stand mixer bowl. In the bowl, add the sugar, coconut oil, butter, soda, powder, salt and vanillas. Affix the paddle attachment and beat until very light and fluffy, should take about 4-5 minutes.
  • Add the egg whites, one at a time, beating between each addition. Continue beating for a few minutes until mixture is very creamy. Scrape down the bowl as needed.
  • Add cake flour (sift it in if lumpy) and turn mixer on low then drizzle in the buttermilk in. Beat on medium until all ingredients are incorporated.
  • Weigh the bowl and the batter on your scale, then subtract the weight of the bowl so you know the weight of the batter. Divide it by 5 or six, depending on how many colors you'd like to do. Now divide the batter into each bowl so they all hold equal amounts. Add food coloring to each bowl and stir to blend.
  • Turn your oven broiler on low.
  • Spread the first color into the bottom of the prepared pan with an offset spatula. It will be a thin layer but it should cover the bottom.
  • Set it in the oven and broil it for 4 minutes.
  • Remove from the oven and spread the second color over the now baked first layer. Return it to the oven for another 4 minutes if you've divided into 6 colors, or 5 minutes if you'v divided into five (thicker layers need to bake longer).
  • Repeat until you have broiled your sixth layer. Now cover the pan with foil and turn the oven to 350. Bake the cake for about 7-10 minutes, until it's thoroughly baked.
  • Allow the cake to cool in the pan until you can handle it then set a cooling rack over it and flip it over. Let it cool completely on the rack. Prepare a pullman loaf pan with parchment paper.
  • Cut off the edges of the cake and slice the remaining rectangle it into 1” strips. The strips should be as long as your loaf pan.
  • Think of the buttercream as ‘glue’ - use a thin layer to stick the strips together and arrange the strips in a mix of horizontal and vertical to create a pretty design. You should have around 9 strips total and arrange them in threes, 3 strips per layer. If you are doing horizontal you might be able to fit two on top of each other with two vertical strips on the side. If you have more than 9 strips, snack on the last ones =)
  • Once all the strips are in, cover the top of the loaf pan with parchment paper and set it in the fridge so the buttercream will firm up and the cake will be easy to frost.
  • Remove the cake loaf from the fridge and, cover the top and sides with buttercream. Decorate as you please.

Video

Notes

I used a recipe I adapted ages ago from Stella Parks to make the white cake. If you have or prefer another white cake recipe, feel free to use it. I do however recommend a white cake and not a yellow as the colors will turn out more accurate.
I suggest you make your buttercream first because the process of baking this cake is so involved (in and out, in and out) so you can’t do it during. Another reason is that if you’ve made the buttercream first, considering it’s the same ingredients in your cake… you won’t have to wash the bowl of your stand mixer! Simply wipe it down to remove any chunks.
The buttercream is going to act like glue; holding the cake parts together. When you start building the loaf with the cake strips, imagine you are playing a game of tetris; vary how you arrange them so that you can get an interesting look when you are done. You will probably need to make another batch of buttercream to cover the sides and top of the cake.