Preheat oven to 350° Spray 2 (9-inch) round cake pans with cooking spray and line bottoms with parchment paper and spray.
Stir together milk and vanilla and reserve.
1 cup Milk, 2 teaspoon Vanilla extract
Beat butter at medium speed with an electric mixer (a stand mixer works best) until creamy, let the mixer run for several minutes until it has creamed to a pale color. Gradually add the sugar, beating until light and fluffy.
1 cup Butter, 2 cup sugar
Sift together flour and baking powder and add to the butter mixture, alternately with the reserved milk mixture, beginning and ending with flour mixture. Beat at low speed just until blended after each addition.
3 cup Cake flour, 1 tablespoon Baking powder
In a separate bowl, beat the egg whites at medium speed until stiff peaks form. Gently fold into the batter. Divide the batter equally into the prepared pans.
6 Egg whites
Bake for 23 to 25 minutes or until a wooden pick inserted in center comes out clean. Cool in pans on wire racks 10 minutes. Turn the cakes out onto wire racks, remove the parchment paper then, with another wire rack on the bottom side of one of the cakes, invert again so the cake is top side up on the rack. Repeat with the second cake. Cool completely before filling and frosting.