Martha's Black & White Brownies

2 pkgs. Walnut brownie mix
1/4 c. butter or margarine
2 c. powdered sugar
4 tsp. milk
1 c. milk chocolate frosting

Preheat oven to 350 degrees. Grease bottom only of 9x13 glass pan. Prepare 2 brownie mixes as directed on package. Pour into greased pan. Bake approx. 40 minutes or until done. Completely cool in pan.
Beat butter until smooth in large mixing bowl. Stir in powdered sugar and milk. Beat until smooth and of spreading consistency. Spread over brownies; chill. Top with frosting; chill 2-4 hours.
Cut into bars and serve at room temperature. Enjoy!!