Prepare cake mix according to package directions, using two 9-in. round baking pans. Cool completely after baking.
If using whipping cream, place it in a chilled bowl; beat, gradually adding sugar and cocoa, until stiff peaks form. If using whipped topping, place in a bowl and fold in sugar and cocoa. Frost one cake layer with whipped cream mixture; top with second layer and frost entire cake. Refrigerate 24 hours before serving.