Line a 9-in. round pan with plastic wrap. Spread ice cream into pan. Freeze 2 hours or until firm.
Prepare and bake cake mix according to package directions, using two 9-in. round baking pans. Cool in pans 10 minutes before removing to wire racks to cool completely.
Using a serrated knife, trim tops of cakes if domed. Place 1 cake layer on a serving plate. Invert ice cream onto cake layer; remove plastic wrap. Top with remaining cake layer. Spread whipped topping over top and sides of cake. Decorate with sprinkles as desired. Freeze 2 hours longer or until firm.