To make a collar for a 1-quart souffle dish, cut a 24-inch length of foil. Fold lengthwise into thirds, making a 4-inch-wide strip. Spray one side of strips with nonstick coating. Place strip, coated side in, around the outside of the souffle dish so it extends about 2 inches above the rim. Pull tight and secure with tape at the point the foil overlaps.
In a medium saucepan combine cocoa powder, 1/4 cup granulated sugar, and cornstarch. Add evaporated skim milk all at once. Cook and stir over medium heat until mixture is thickened and bubbly. Cook and stir 2 minutes more. Stir some of the cocoa mixture into the egg product. Then return all of the egg product mixture to the saucepan. Bring to a gentle boil. Remove from heat. Stir in rum or rum flavoring.
In a large mixing bowl beat the egg whites and cream of tartar with an electric mixer on medium speed until soft peaks form (tips curl). Add 1/4 cup granulated sugar, 1 tablespoon at a time, beating on high speed until stiff peaks form (tips stand straight). Fold a small amount of beaten egg white mixture into chocolate mixture to lighten it. Then fold chocolate mixture into remaining beaten egg whites. Turn mixture into prepared souffle dish.
Bake in a 350 degree F oven for 35 to 40 minutes or until a knife inserted near the center comes out clean. Remove the collar; sift powdered sugar over top. Serve immediately with Whipped Topping or Whipped Milk Topping. Makes 6 servings.