Combine cookie crumbs and 1/3 cup butter in medium bowl. Press onto bottom of ungreased 9x9 inch baking pan and refrigerate until firm.
Combine powdered sugar, remaining 1/3 cup butter, milk, and creme de menthe in mixing bowl.
Beat at medium speed until light and fluffy (should resemble frosting.) Spread over crust.
Drizzle melted chocolate over crust and refrigerate until firm, about 2 hours.
Cut into bars; store in refrigerator.
(adapted from The Great American Cookie Cookbook's Citrus Cream Bars recipe)