Cream butter and sugar. Sift together flour and baking powder and add to creamed butter. Beat egg whites. Add half of the milk to the batter, then fold in egg whites. Add remaining milk, only as needed to make a moist batter. With an electric mixer, beat for 3 to 4 minutes, then pour into three buttered and floured 9-inch round cake pans. Bake at 350 degrees F for 30 minutes. Frost as desired.