Grease 13x9-inch pan with butter or cooking spray. In large microwavable bowl, microwave 3 tablespoons butter and the marshmallows uncovered on High 2 minutes or until melted and mixture can be stirred smooth. Stir in cereal and peanut butter pieces. Quickly spread in pan.
In small microwavable bowl, microwave chocolate and peanut butter uncovered on High 1 minute 30 seconds to 2 minutes, stirring every 30 seconds, until softened and chocolate can be stirred smooth. Drizzle chocolate mixture over cereal mixture. Sprinkle with peanut butter cup candies. Let stand 30 minutes until chocolate is set. Cut into 6 rows by 4 rows.