Preheat oven to 350 degrees. Grease or spray the bottom only of a 8 x 8-inch square pan.
Cream butter and sugar until light and fluffy. Add eggs, one at a time, beating well after each addition; add milk and beat until incorporated.
Mix flour and baking powder and stir into butter/egg mixture. Remove half of the batter to a separate bowl. Stir cocoa into one half of the batter. Add mint extract and food coloring to other half of the batter.
Drop batters by heaping tablespoonfulls alternating chocolate and green batters in marble cake fashion. Lightly swirl batter in pan.
Bake at 350 degrees for 25-30 minutes. Let cool.
Prepare ganache frosting by microwaving chocolate chips and heavy cream(start with 1 tbs of cream, you can add more if necessary) at 1 minute intervals at 50% power; stir. Repeat until frosting is smooth in texture. You should have a thick but pourable consistency. Pour on top of brownies and spread evenly. Let ganache set--about 1 hour. Makes 9 brownies.