Sift together corn starch, pastry flour, baking powder and salt. Separate the eggs. Beat the egg whites until stiff and set aside. In a large bowl, cream the shortening and the sugar. Add the egg yolks, and beat until mixture is very creamy. Sift the dry ingredients, and add alternately with the milk. Stir in the vanilla. Fold in the beaten egg whites. Place in a greased 9-inch square pan. Bake at 350 degrees F. for 45-50 minutes. Cool cake on a rack and frost with lemon butter icing.