Sift together the cornstarch, 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 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. Bake in a greased 9 inch square pan.
Bake at 350 degrees F (175 degrees C) for 45 to 50 minutes. Cool cake on a rack, and frost with a lemon butter icing.