Sift flour, baking powder, and salt together. Melt butter in an 8 by 8-inch cake pan, add maple syrup remove from heat. Place apple slices in rows in syrup mixture. Cream shortening and 1/2 cup sugar together, add egg yolks, beat well. Add dry ingredients and milk alternately in thirds, beating after each addition. Stir in grated apples. Beat egg whites until frothy; beat in remaining sugar gradually until stiff peaks form. Fold into batter; spread over apples. Bake at 350F about 45 minutes. Cool 5 minutes and invert on plate.