Chocolate Angel Food Cake

8 servings. Preheat oven to 375F. Place sugar and cake flour in a small bowl. Sift 1/4 cup cocoa powder over and mix well; set aside. In a large mixer bowl at high speed, beat egg whites until foamy. Add cream of tartar and continue beating until stiff but not dry. Sprinkle 1 teaspoon vanilla over egg whites. Fold in cocoa mixture, half at a time, until well blended. Spread evenly in an ungreased 10-inch tube pan. Bake in lower third of oven for 30 minutes or until top springs back when touched. Invert and cool in pan. Prepare whipped topping according to package directions, substituting skim milk for whole milk and adding 1 tablespoon cocoa powder with vanilla. Frost cake, reserving about 1/2 cup for piping on a decoration if desired. Sprinkle with additional cocoa powder.