This recipe is used in our Heavenly Creme Brulee Cake.
Preheat oven to 350 degrees. Brush two 8-inch cake pans with butter. Line with parchment paper; butter parchment, and dust with flour, tapping out any excess. Set aside.
Whisk together flour, baking powder, and salt in medium bowl; set aside. Stir milk and vanilla to combine; set aside. Beat butter in a mixer bowl until pale and fluffy, about 3 minutes. Add sugar in steady stream; mix until pale and fluffy, about 3 minutes.
Reduce the speed to low. Add flour mixture in 3 additions, alternating with milk mixture, and beginning and ending with flour; mix just until combined.
In a clean mixing bowl, whisk egg whites until stiff peaks form. Fold one-third of egg whites into batter to lighten. Gently fold in remaining whites in two batches. Divide batter among prepared pans; smooth tops with an offset spatula. Firmly tap pans on work surface to release any air bubbles.
Bake until a cake tester inserted into centers comes out clean and tops are springy to the touch (see below for baking times). Let cool in pans on wire racks for 15 minutes, and then invert cakes onto racks. Remove parchment; reinvert and cool completely.