Mix and spread in well greased 9 x 13 inch pan. Bake at 350 degrees for 30-35 minutes. Cool.
Cream together 2 cups powdered sugar, 1/2 cup butter, 2 Tbsp. Milk, and 1/2 tsp. peppermint extract and a few drops of green food coloring. Frost cooled brownies. Melt 1 cup chocolate chips and 6 Tbsp. Butter. Pour over frosting. Cool until glaze hardens.