1Line 9 x 9-inch pan with foil so foil extends about 2 inches over the sides of pan. Spray foil with cooking spray. In large microwavable bowl, microwave butter and marshmallows uncovered on High about 2 minutes, stirring after each minute, until mixture is smooth. Remove from microwave.
2Immediately stir in cereal until evenly coated. Pour cereal mixture into pan. With buttered back of a spoon, press mixture in pan until even. Cool at least 10 minutes.
3Using foil to lift, remove brownies from pan. With mustache shaped cookie cutter, cut cereal mixture into 24 bars. Store in loosely covered container.