Prepare brownie mix according to package directions but add 1 T creme de menthe to batter before mixing. Let cool completely.
Mix powdered sugar, 1/2 cup of butter and 2 T creme de menthe together until combined. Add 3-6 drops of green food coloring to achieve desired color and mix well until well blended.
Spread green frosting layer evenly over brownies. Chill for 30 minutes.
Melt 6 T butter and chocolate chips in small saucepan over low heat, stirring continuously until chips are melted and sauce is smooth. Pour chocolate mixture over green frosting layer and spread evenly to form the top layer.