Prepare brownie mix according to package directions, adding mint extract to batter. Transfer to a greased 13x9-in. baking pan. Bake at 350° for 20-25 minutes or until a toothpick inserted in the center comes out clean. Cool completely on a wire rack.
Meanwhile, in a small bowl, cream the butter, confectioners' sugar, vanilla and, if desired, food coloring until light and fluffy. Transfer to a pastry bag. Set aside.
Line a baking sheet with parchment. Dust with cocoa; set aside. Cut brownies into 15 rectangles. Using a 1-1/2-in. heart-shaped cookie cutter, cut out a heart from the center of each brownie. Reserve cutout centers for another use. Place brownies on prepared baking sheet. Pipe frosting into centers of brownies.