Prepare brownies according to package directions for cake-like brownies. Cool completely. Combine powdered sugar, 1 stick margarine, water, mint extract and green food coloring in mixing bowl; beat until creamy. Spread over brownies; refrigerate until set. Melt chocolate chips and 9 TBSP margarine in microwave, checking and stirring at 3o-second intervals. Let cool until lukewarm, stirring occasionally. Spread over filling. Chill in fridge before cutting into squares.