Combine egg whites (at room temperature) and cream of tartar in a large bowl.
Beat egg whites at high speed of electric mixer until stiff peaks form; set egg whites aside.
Cream shortening, gradually add sugar, beating at medium speed of an electric mixer until well blended.
Combine cake flour, soda, and salt, add to creamed mixture alternately with buttermilk, beginning and ending with flour mixture.
Stir in flavorings.
Fold in one-fourth of beaten egg whites, blend well.
Fold in remaining egg whites.
Pour batter into 3 greased and floured 9-inch round cakepans.
Bake at 350° for 25 minutes or until a wooden pick inserted in center comes out clean.
Cool cake layers in pans 10 minutes, remove from pans, and cool completely on wire racks.
Spread Lemon Filling between layers, spread top and sides with Fluffy Frosting.
Garnish with lemon slices, if desired.