1. Preheat oven to 350 degrees. Butter and line two 9-inch round cake pans with parchment. Butter parchment, then flour pans, tapping out excess. Set aside.

2. Whisk together flour, baking powder, baking soda, and salt in a medium bowl; set aside. Put butter and sugar in the bowl of an electric mixer fitted with the paddle attachment; cream on medium-high speed until pale, 2 to 3 minutes. On medium speed, add eggs yolks one at a time, mixing well after each addition. Add vanilla and mix, scraping down sides of bowl. On low speed, add the flour mixture in three batches, alternating with two batches of buttermilk. Fold in egg whites. Stir with a rubber spatula until the batter is evenly blended.

3. Divide batter between pans. Smooth the top of each layer with a small offset spatula.

4. Bake until a cake tester inserted into centers comes out clean, 30 to 35 minutes.

5. Remove from oven and let cool in pans on wire racks, 20 minutes. Run a knife around edges of cakes to loosen. Invert cakes from pans. Peel off parchment from cakes. Reinvert and let cool completely on racks.