HEAT oven to 350°F.
LINE 8-inch square pan with foil, with edges of foil extending over sides; spray with cooking spray. Microwave chocolate and butter in large microwaveable bowl on HIGH 2 min. or until butter is melted. Stir until chocolate is completely melted and mixture is well blended. Add sugar; mix well. Blend in eggs. Stir in flour and cocoa powder.
SPREAD 1/3 of the batter onto bottom of prepared pan. Arrange mint patties in single layer over batter in pan; cover with remaining batter.
BAKE 30 to 35 min. or until toothpick inserted in center comes out with fudgy crumbs. (Do not overbake.) Use foil handles to lift brownies from pan before cutting to serve.