- 1 cup unsalted butter, at room temperature, plus extra for greasing
- 2 1/4 cups all-purpose flour
- 1/3 cup cornstarch
- 4 1/2 tsp. baking powder
- 1/2 tsp. salt
- 2 cups superfine sugar
- 2 tsp. vanilla extract
- 4 medium eggs, separated
- 1 1/4 cups whole milk
- Gel food coloring in red, yellow, orange, purple, blue and green
For the white chocolate Italian meringue frosting:
- 5 1/2 ounces white chocolate
- 1 1/2 cups superfine sugar
- 6 medium egg whites
- 2 cups + 6 Tbsp. unsalted butter, at room temperature
Preheat the oven to 350°F, then grease and line three 8-inch round cake pans with parchment paper, greasing the parchment, too. In a medium bowl, whisk the flour, cornstarch, baking powder and salt together to combine, then set aside.
Put the butter in a large bowl and, using an electric mixer, beat on medium-high speed until smooth and light. Add the sugar and beat until light and fluffy, about 5 minutes, then add the vanilla and mix to combine. With the mixer on medium speed, add the egg yolks, one at a time, beating each until fully combined before adding the next. With the mixer on low, sift in the flour mixture in three additions, alternating with the milk, starting and finishing with the flour.
Pour the egg whites into a clean, grease-free bowl and whisk until they hold soft peaks. Add the whites to the cake batter and gently fold together until just combined and there are no streaks of white. Divide the batter equally between six small bowls. Add a small amount of each coloring to each bowl, enough to make it a vibrant color. Add the first three bowls to the prepared pans and bake for 15 minutes or until they spring back when lightly touched. Let cool in the pans for 10 minutes before turning onto a wire rack to cool completely. Repeat with the remaining three portions of batter.
To make the frosting, melt the chocolate in a heatproof bowl set over a pan of gently simmering water, making sure the bottom of the bowl doesn't touch the water. Remove from the heat and let cool.
Put 2/3 cup water and the sugar in a pan over medium heat. Bring to a boil and have a sugar thermometer ready. Meanwhile, pour the egg whites into a clean, grease-free bowl (this is best done using a freestanding electric mixer). Start beating the whites on high speed when the syrup in the pan reaches about 240°F on the sugar thermometer. Cook until the syrup registers 250°F. With the mixer still running, pour the syrup in a slow stream down the side of the bowl containing the whites, avoiding the beaters. Continue beating on high speed until the meringue is at room temperature. With the mixer on medium-high speed, add the butter a few pieces at a time, beating until fully combined. When adding the butter it can sometimes look curdled; if this happens, don't worry, just keep mixing and eventually it will smooth out to form a light buttercream frosting. With the mixer on medium speed, pour in the melted chocolate and mix until fully combined.
To assemble the cake, put the purple cake layer on a cardboard cake board or serving plate and top with a thin layer of frosting. Repeat the process with the other cake layers in the order of a rainbow (purple, blue, green, yellow, orange, red) and then spread the remaining frosting across the top and sides of the cake.
From Say It with Cake by Edd Kimber © 2013 Kyle Books.
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