Prepare and cool the Lemon Filling. Grease and lightly flour three 8x1-1/2-inch round baking pans. Set aside. In a medium mixing bowl combine flour, baking powder, and salt.

In a large mixer bowl beat butter or margarine and shortening with an electric mixer on medium speed for 30 seconds. Gradually add sugar and vanilla, beating until light and fluffy. Add egg yolks, one at a time, beating 1 minute after each.

Add dry ingredients and milk alternately to beaten mixture, beating after each addition until blended. Gently fold in egg whites by hand.

To make the 6 cake layers, transfer a generous 3/4 cup of batter into each prepared pan, spreading evenly over the bottom of pan. Bake in a 375 degree F oven for 10 to 12 minutes or until cakes test done. Cool 5 minutes in pans. Remove from pans and cool on wire racks.

Wash, grease, and flour pans. Spoon the remaining batter into the 3 pans. Bake and cool as directed above.

To assemble, place 1 cake layer on serving plate. Spread a scant 1/2 cup Lemon Filling evenly atop. Repeat with 4 more layers and filling. Top with final layer. Cover and chill thoroughly. Spread Lemon Icing over top and sides of cake. Cover and store in the refrigerator. Before serving, sprinkle top with shredded lemon peel, if desired. Makes 16 servings.

In a saucepan combine 1-1/4 cups sugar, 3 tablespoons cornstarch, 3 tablespoons all-purpose flour, and dash salt. Stir in 1-1/2 cups cold water. Cook and stir 2 minutes more. Stir in 2 tablespoons butter or margarine and 1/2 teaspoon finely shredded lemon peel. Gradually stir in 1/3 cup lemon juice. Mix well. Cover surface with clear plastic wrap. Cool the mixture to room temperature without stirring. Stir about 1 cup of the hot mixture into 3 beaten egg yolks. Return mixture to saucepan. Bring mixture to a gentle boil. Cook and stir 2 minutes more. Remove from heat.

In a mixer bowl beat two softened 3-ounce packages cream cheese, and 3 cups powdered sugar until fluffy. Add 1 teaspoon finely shredded lemon peel and 1/4 teaspoon vanilla and beat until smooth.