I have made this cake many times in the past. Its so good. I got the recipe from a friend of mine at work, Kathy. In the past, I always made it the "easy way," as I've described below. But this past weekend, I made a white cake from scratch (I doubled that recipe in the link to do a 9x13) and just added the creme de menthe to that, rather than using a boxed cake mix. I also made my own hot fudge. I did use a tub of frozen whipped topping from Whole Foods, rather than making my own whipped cream... I did think about making it, but decided against it. It turned out even more delicious making most of it from scratch!
I'll try to remember to take a picture of some of the leftover cake that we have at home.
Creme de Menthe Cake
1 white cake mix
1 jar fudge ice cream topping
1 container Cool Whip
Creme de Menthe liquor
Prepare the cake mix as directed (or make your own from scratch), taking 3-4 tablespoons of water out and replacing with creme de menthe. Bake as directed.
Soften the hot fudge enough to spread on top of the cake (or make your own from scratch).
Add 3-4 teaspoons of creme de menthe to the whipped topping, stir. Spread on top of cake. Its usually easier to spread the Cool Whip if you let the fudge set up a bit.
Chill in the refrigerator until you are ready to serve.