In a bowl, combine crushed graham crackers, 1/3 cup melted butter or margarine and granulated sugar. Press into the bottom of a 15x10x1-inch baking pan. Set in freezer to chill for 5 minutes.
Cut ice cream into 1/2-inch-thick slices and lay over the crumbs. Cover and return to freezer.
For chocolate sauce, in a saucepan, melt chocolate and 1/3 cup butter or margarine. Add milk and powdered sugar. Cook and stir for about 20 to 25 minutes until slightly thickened. Stir in vanilla. Cool sauce completely.
Pour chocolate sauce over mint-chocolate chip ice cream and return to freezer for about 30 minutes or until firm.
Spread thawed dessert topping on top; freeze again until serving time. Makes 32 servings.