1Move oven rack to lowest position (remove other racks). Heat oven to 350°F. In extra-large glass or metal bowl, stir cocoa into dry cake mix; add water. Beat with electric mixer on low speed 30 seconds. Beat on medium speed 1 minute. Pour into ungreased 10-inch angel food (tube) cake pan. (Do not use fluted tube cake pan or 9-inch angel food pan or batter will overflow.)
2Bake 37 to 47 minutes or until top is dark golden brown and cracks feel very dry and not sticky. Do not underbake. Immediately turn pan upside down onto glass bottle, or rest edges on 2 to 4 cans of equal height until cake is completely cooled, about 2 hours. Run knife around edge of cake; remove from pan.
3Drain raspberries, reserving liquid. Add enough water to raspberry liquid to measure 2/3 cup. In 1-quart saucepan, mix sugar and cornstarch; stir in liquid mixture. Heat over medium heat, stirring constantly, until mixture thickens and boils; boil and stir 1 minute. Stir in orange juice and raspberries. Cool completely.
4Split cake horizontally to make 3 layers. (To split, mark side of cake with toothpicks and cut with long, serrated knife.) Spread 1 cup whipped topping and scant 1/2 cup sauce between each layer (sauce may not completely cover each layer). Spread remaining sauce over top of cake. Drop remaining whipped cream in dollops on top of sauce. Garnish with mint leaves, if desired. Store in refrigerator.