Sift flour once, measure, add baking powder and salt, and sift together three times. Cream shortening, add sugar gradually, and cream together until light and fluffy. Add flour, alternately with milk, a small amount at a time, beating after each addition until smooth. Add vanilla. Beat egg whites until they will hold up in moist peaks. Stir quickly but thoroughly into batter. Bake in two greased 8-inch layer pans in moderate oven (375 degrees F.) 25 minutes or until done. (All measurements level)