Prepare cake mix according to package directions, baking in two 9-inch round baking pans. On wire racks, cool 15 minutes. Remove cakes from pans and cool completely. Trim top of one cake to flatten. Wrap cakes in aluminum foil and freeze at least 2 hours.
Remove cakes from foil and on serving platter, arrange trimmed cake, flat side down. Spoon Breyers® Homemade Vanilla Ice Cream onto cake, evenly spreading to the sides. Top with second cake, pressing down slightly. Freeze 5 hours or overnight.
To serve, microwave heavy cream 30 seconds or until hot; stir in chocolate until smooth. Let cool 5 minutes; evenly pour over cake. Serve immediately or freeze until ready to serve.