Make cake according to box directions; substituting 1/4 cup of Creme de Menthe for 1/4 of water to the mix.
Bake in a 9x13" cake pan.
Spread the hot fudge topping on top.
Mix some of the Creme de Menthe in the thawed Cooll Whip. (enough to tint green) and to give a little flavor. Not alot, otherwise, it will be too overpowering and will dilute the Cool Whip. So be careful.
Spread this over the fudge topping layer.
Cover and chill over night.
****IMPORTANT TIP: warm the Hot Fudge in microwave so that it spreads over the cake easy without tearing up the cake. Also, for the Cool Whip, only put enough Creme de Menthe in the Cool Whip to give a bit of flavor. Add green food coloring for the added green color.