Sift flour once, measure, add baking powder and sift together three more times. Cream shortening thoroughly, add sugar gradually, and cream together until light and fluffy. Add egg yolks and beat well. Add extract. Add sifted flour and baking powder, alternately with milk, a small amount at a time. Beat after each addition until smooth. Bake in layers in moderate oven (325 degrees F.) 25 minutes. Makes three 9-inch layers.