You can use either Creme de Menthe Syrup, which is an ice cream topping, or Creme de Menthe Liquor. I have used both with equal success in this recipe. The Syrup can be found in some grocery stores with the rest of the ice cream toppings. The cake will be slightly lighter green in color and will have a slightly milder mint flavor. The syrup is a little sweeter that the liquor, so the whipped topping will have a sweeter taste. The Liquor creates a slightly more vivid colored cake and stronger mint flavor with a bit less sweetness. Both options are delicious. I usually go with whichever option I find first.