Preheat your oven to 350°F (175°C). Grease and flour your cake pans.
In a mixing bowl, cream together the butter and sugar until light and fluffy.
Add the egg whites one at a time, beating well after each addition.
Mix in the vanilla extract.
In another bowl, sift together the flour, baking powder, and salt.
Alternately add the flour mixture and milk to the butter mixture, beginning and ending with the flour mixture. Mix until just combined.
Pour the batter into the prepared pans and bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cakes to cool in the pans for 10 minutes, then remove from pans and cool completely on a wire rack.