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
2 teaspoons baking powder
1/2 teaspoon baking soda
1/2 teaspoon kosher salt
12 tablespoons (168 g) unsalted butter, at room temperature
1 1/2 cups (300 g) granulated sugar
1 tablespoon clear vanilla flavoring (I used McCormick brand)
6 egg whites (150 g), at room temperature
1 1/3 cups (10 2/3 fluid ounces) buttermilk, at room temperature
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. Set the pans aside. Into a medium-size bowl, sift (yes, you have to sift!) the flour, xanthan gum, cornstarch, baking powder, and baking soda. Add the salt, and whisk to combine well. Set the dry ingredients aside.
In the bowl of a stand mixer fitted with the paddle attachment or a large bowl with a handheld mixer, beat the butter until it’s pale in color. Add the sugar, vanilla flavoring and egg whites, and beat on medium-high speed for about a minute or until light and fluffy, stopping at least once to scrape the entire mixture off the sides and bottom of the bowl. To the large bowl with the butter and sugar mixture, add the dry ingredients in 3 equal portions, alternating with the buttermilk in 2 parts, beginning and ending with the dry ingredients and mixing to combine in between additions. Once all of the ingredients have been added, beat with the mixer until mostly combined, then turn over the batter a few times by hand with a spatula. It should be very thickly pourable and relatively smooth.
Divide the batter evenly between the two prepared baking pans and smooth each into an even layer with an offset spatula. Smack each pan flat on the counter a few times to break up any large pockets of air. Place the baking pans in the center of the preheated oven and bake for 20 minutes. Rotate the pans, and continue to bake until the cakes are lightly golden brown all over, have begun to pull away from the sides of the pan (about another 10 minutes). If the cakes have domed in the oven, place a clean oven mitt on top of each of the pans and press down gently to help flatten the tops. Allow the cakes to cool in the pans for 15 minutes before turning out onto a wire rack (and removing the parchment paper liners) to cool completely before frosting and serving.