Preheat your oven to 350°F. Grease and flour two 9-inch round cake pans.
In a bowl, whisk together the flour, baking powder, and salt. Set it aside.
In a large mixing bowl, beat the butter and sugar until it looks pale and fluffy.
Add the egg whites, one at a time, mixing well after each.
Stir in the vanilla extract.
Add the dry ingredients in three batches, alternating with milk. Start and end with dry ingredients.
Mix until just combined. Don’t overmix, unless you enjoy dense cake (we don't).
Pour the batter evenly into the prepared pans. Smooth the tops with a spatula.
Bake for 25-30 minutes, or until a toothpick comes out clean from the center.
Let the cakes cool in the pans for 10 minutes before turning them out onto a wire rack to cool completely.