For the cake
2 cups (280 g) all purpose gluten free flour (I used Better Batter)
1 teaspoon xanthan gum (omit if your blend already contains it)
1/4 cup + 2 tablespoons (54 g) cornstarch
1/2 teaspoon baking soda
2 teaspoons baking powder
1/2 teaspoon kosher salt
1 1/3 cups (10 2/3 fluid ounces) buttermilk, at room temperature
4 egg whites (100 g) + 1 whole egg (50 g, weighed out of shell), at room temperature
2 teaspoons pure vanilla extract
10 tablespoons (140 g) unsalted butter, at room temperature
1 1/2 cups (300 g) granulated sugar
3 tablespoons (30 g) edible gluten free rainbow confetti, plus more for decorating
For the chocolate icing
6 tablespoons (84 g) unsalted butter, chopped
1 1/8 cups (125 g) granulated sugar
7/8 cup (70 g) unsweetened cocoa powder (Dutch-processed is best, but natural works)
1/8 teaspoon kosher salt
1 cup (8 fluid ounces) heavy whipping cream
6 tablespoons (90 g) full fat sour cream
1 teaspoon pure vanilla extract
For the chocolate sour cream frosting
12 tablespoons (168 g) unsalted butter, at room temperature
1/2 cup (40 g) unsweetened cocoa powder (natural or Dutch-processed is fine)
4 cups (460 g) confectioners’ sugar
1 1/2 teaspoons kosher salt
3/4 cup (180 g) full fat sour cream, at room temperature
1 teaspoon pure vanilla extract
You may want to make the chocolate icing in step 5 below before you even make the cake. It is easy to make and has to cool before you can use it.
First, make the cakes. Preheat your oven to 350°F. Grease 2 8-inch round cake pans and line the bottom of each with a round of parchment paper (trace the perimeter of the cake pan on the parchment, then cut out the circle). Set the pans aside.
Into a medium-size bowl, sift the flour, xanthan gum, cornstarch, baking soda, and baking powder. Add the salt, and whisk to combine well, then set the dry ingredients aside. In a measuring cup or bowl with a pour spout, combine the buttermilk, egg whites, egg, and vanilla, whisk to combine well and set it aside. In the bowl of a stand mixer fitted with the paddle attachment or a large bowl with a handheld mixer, beat the butter and sugar on medium-high speed for at least 3 minutes, stopping at least once to scrape the entire mixture off the sides and bottom of the bowl, or until very light and fluffy. To the large bowl with the butter and sugar mixture, add the dry ingredients in 4 equal portions, alternating with the buttermilk and egg mixture in 3 parts, beginning and ending with the dry ingredients, mixing to combine in between additions. The batter should be very thickly pourable and relatively smooth. Add the edible confetti, and beat once more until the confetti is evenly distributed throughout the batter.
Divide the batter evenly between the two prepared baking pans and smooth each into an even layer with an offset spatula. Tap the bottom of each pan on the counter a few times to ensure that the batter is evenly distributed on the bottom of the pan. The edible confetti has a tendency to create pockets in the batter, which can cause the cakes to bake unevenly. Place the baking pans in the center of the preheated oven and bake for 20 minutes. Rotate the pans, and continue to bake until they’re lightly golden brown all over, have begun to pull away from the sides of the pan, and do not jiggle in the center at all (about another 10 minutes). Remove the cakes from the oven and allow to cool in the pans for 15 minutes before turning out onto a wire rack (and removing the parchment paper liners) to cool completely.
While the cakes are cooling, make the frosting and the icing. For the chocolate icing, place the butter in a large, heavy-bottom saucepan and melt over medium-low heat. Add the sugar, cocoa powder, and salt, then the cream and sour cream, and whisk until as smooth as possible. Reduce the heat to low and continue to cook, whisking frequently until the mixture is very smooth and no longer grainy at all. Do not let it boil. Remove the icing from the heat and whisk in the vanilla until well-combined. Transfer the mixture to a heat-safe bowl and allow it to cool until no longer hot to the touch. Cover and refrigerate until it holds its shape when scooped (at least 30 minutes).
For the frosting, in the bowl of a stand mixer fitted with the paddle attachment or a large bowl with a handheld mixer, whip the butter until light and fluffy. Add the cocoa powder, confectioners’ sugar, salt, sour cream, and vanilla, and beat on medium speed until the powder is absorbed by the butter. Turn the mixer speed up to high and beat until very well-combined and light and fluffy but holds its shape. Set the frosting aside.
When the cakes and the chocolate icing are all cool, assemble the cake. Place one of the cake layers flat on a rotating cake stand if you have one, or a cake plate if you don’t. Line the edges of the plate or stand with 3 pieces of overlapping parchment to protect the surface from drips. Place about 1 1/2 cups of the chocolate frosting on top of the first cake, and spread into an even layer on top, and continue a thin layer around the side of the first layer. Place the second cake on top, and cover with the remaining chocolate frosting. Spread the frosting into an even, thin layer all around the cake, using it to fill in any gaps in between the sides of the two cakes. You can chill the cake at this point to ensure that the icing goes on smoothly, but I often don’t. When ready, pour the cooled chocolate icing on top of the cake, and use it to cover the top and sides of the cake. Decorate with additional edible confetti, if you like. Slice and serve.