1Move oven rack to lowest position (remove other racks). Heat oven to 350°F. In extra-large glass or metal bowl, beat cake mix, cocoa and water 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 until cake is completely cool, about 2 hours.
3Run knife around side of pan to loosen cake; remove from pan. Cut off top of cake about 1 inch from top; set aside. Cut down into cake 1 inch from outer edge and 1 inch from edge of center hole, leaving substantial "walls" on each side. Remove cake within cuts with curved knife or spoon, being careful to leave a base of cake 1 inch thick. Spoon ice cream into cake cavity; smooth top. Replace top of cake. Cover and freeze about 3 hours or until firm. Serve with chocolate-flavor sauce.