Make the Nutella Mousse: Whip heavy cream then mix together with 1/4 cup Nutella. (If using Cool Whip, mix together container of Cool Whip and Nutella until well combined). Place a layer of chocolate wafer cookies on the bottom of a bread-size baking container or small casserole dish. Spoon first layer of Nutella mousse over top. Place Klondike Bars over top of mousse, filling in all spaces with sliced bars. Spoon softened vanilla ice cream over top of bars. Add second layer of chocolate wafer cookies. Slather second layer of mousse over top. Add last layer of softened ice cream. Curl or shave chocolate bar with knife or potato peeler. (Chocolate curls better if slightly warm). Spread over the top. Freeze cake for several hours or best if frozen overnight. Slice or spoon onto serving dishes.