Sift flour once, measure, add baking powder and sift together three times. Cream butter thoroughly, add sugar gradually and cream together until light and fluffy. Add egg whites, 1/4 cup at a time, beating well after each addition. Add flavoring and beat vigorously. Turn into two loaf pans 9x5x3 inches, which have been greased, lined with heavy paper and again greased. Bake in slow over (275 degrees F) 40 minutes; then increase heat lightly (325 degrees F) and bake 40 minutes.