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
10 tablespoons (140 g) unsalted butter, at room temperature
1 1/2 cups (300 g) granulated sugar
2 teaspoons pure vanilla extract
4 egg whites (100 g) + 1 whole egg (50 g, weighed out of shell), at room temperature
1 1/3 cups (10 2/3 fluid ounces) buttermilk, at room temperature
*A note about flours: If you are using a higher starch all purpose gluten free flour blend like Cup4Cup (or my mock Cup4Cup), replace the cornstarch in this recipe with an additional 54 grams of your flour blend.
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 (yes, you have to sift!) the flour, xanthan gum, cornstarch, baking soda and baking powder. 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, sugar and vanilla 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. Combine the egg whites, egg and buttermilk in a small bowl, and whisk to combine well. 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 and mixing to combine in between additions. The batter will sometimes look a bit curdled. Once all of the ingredients have been added, beat for another minute on medium speed to ensure that everything is combined, then turn over the batter a few times by hand. 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. 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 and do not jiggle in the center at all (about another 10 minutes). I find that these tests for doneness are more useful than the toothpick test.
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 before frosting and serving. I frosted with 3 cups of a cream cheese buttercream frosting.