Prepare cake mix according to package directions using two 8- or 9-inch round cake pans, except replace 1/4 cup of the water with creme de menthe. Remove cakes from pans and cool as directed. If necessary, cut a thin slice off the top of each cake to make the tops flat.
Place ice cream topping in a small microwave-safe bowl. Micro-cook on 50% power (medium) for 30 to 40 seconds or until spreadable, stirring once. Spread ice cream topping atop bottom of one of the cake layers. Top with remaining cake layer. Combine frosting with remaining 1 tablespoon creme de menthe in a medium bowl. If using clear creme de menthe, tint frosting with a few drops of green food coloring, if desired. Frost top and sides of cake and garnish with chocolate-mint candy, if desired.