In an extra-large mixing bowl allow egg whites to stand at room temperature for 30 minutes. Meanwhile, sift powdered sugar and flour together 3 times. Measure 1 cup of the flour mixture; sift together with the cocoa powder. Set mixtures aside.

Add cream of tartar and vanilla to the egg whites. Beat with an electric mixer on medium speed until soft peaks form (tips curl). Gradually add the granulated sugar, about 2 tablespoons at a time, beating until stiff peaks form (tips stand straight). Transfer one-third of the mixture to another bowl.

Sift about one-fourth of the plain flour mixture over the larger portion of beaten egg whites; gently fold in. Repeat folding in the remaining plain flour mixture, using one-fourth of the flour mixture each time. Sift flour-cocoa mixture, one-third at a time, over small portion of beaten egg whites, folding in as above.

Gently spoon one-third of the white batter evenly in an ungreased 10-inch tube pan. Spoon half of the chocolate batter over white batter in pan. Repeat. Top with remaining white batter. Marble the batter.

Bake on the lowest rack in a 350 degree F oven for 40 to 45 minutes or until top springs back when lightly touched. Immediately invert cake (leave in pan). Cool completely. Using a narrow metal spatula, loosen sides of cake from pan. Remove cake from pan. If desired, drizzle with Chocolate Powdered Sugar Icing and serve with sweetened whipped cream or fresh fruit. Makes 12 servings.

Combine 1 cup powdered sugar, 1 teaspoon vanilla, and 1 tablespoon milk or juice. Stir in additional milk or juice, 1 teaspoon at a time, until icing is easy to drizzle.

Prepare as above, except add 2 tablespoons unsweetened cocoa powder to powdered sugar. Do not use orange juice.

To marble batter, using a folding motion, bring a narrow spatula down through batter and up other side. Go completely around pan, turning pan as you fold. Do not overfold.