Sift flour, cornstarch and half of the sugar (3/4 cup) together, 3 times. Place egg whites in a large mixing bowl and sprinkle the salt and cream of tartar over the surface. Beat egg whites until stiff but not dry; gradually beat in remaining sugar and vanilla and continue beating until mixture forms stiff peaks but is not dry. Fold flour mixture in gently, 2 tablespoons at a time. Carefully place batter into an ungreased 10 inch tube pan. Bake in moderate oven (375 F.) 25 to 30 minutes or until cake is lightly browned and springs back when lightly touched with finger tips. Invert pan so that cake does not rest on upper crust. A soft drink bottle will easily support the tube pan by inserting the neck of the bottle into the hollow tube. (with most soft drink containers being plastic now a beer bottle works well) Leave inverted until thoroughly cooled: loosen sides and around tube with a flat knife or spatula and remove from pan. Frost if desired. Note: Dry ingredients MUST BE SIFTED together in making Angle Food Cake