Preheat oven to 350°F. In bowl, sift together flour and baking powder. In large bowl, cream butter and sugar together with electric mixer until light and fluffy. Add flour mixture alternately with milk; mix well. Add vanilla. In separate bowl, beat egg whites until stiff but not dry; fold into batter. Pour into prepared pan. Bake 20-25 minutes or until toothpick inserted comes out clean. Cool 10 minutes in pan on rack; remove and cool completely before decorating.