Gluten Free White Velvet Cake (Buttermilk Cake)

Preheat the oven to 350°F. Grease two 8-inch cake pans and line the bottoms with parchment paper for best results.
Sift together the dry ingredients (flour, sugar, cornstarch, baking powder, baking soda, and salt) in a large mixing bowl.
Using a stand mixer with the paddle attachment, or a hand mixer, mix the softened butter into the dry ingredients on low speed until the mixture resembles coarse sand.
In a separate bowl, whisk the egg whites until foamy, then mix in the buttermilk, oil, vanilla, and vinegar.
With the mixer running, slowly pour the liquid ingredients into the flour mixture, allowing each addition to incorporate before adding more. Scrape down the bowl as needed.
Divide the batter evenly between the two pans and let it rest for 15 minutes.
Bake for 35 to 40 minutes, or until a toothpick inserted in the center comes out clean and the cake is just starting to pull away from the sides of the pan.
Cool the cakes on a wire rack for 15 minutes. Use a small offset spatula to loosen the edges, then turn the cakes out onto the rack to cool completely.