Mix cake according to package directions and add the Creme De Menthe. Pour into a greased and floured 9x13 inch pan. Bake at 180ºC for 30-35 minutes. Frost cake with canned frosting. Refrigerate 10 minutes. Mix Cool Whip and 3 T. Creme De Menthe and spread over icing. May decorate with shaved chocolate bar.