1Spray rectangular pan, 13x9x2 inches, with cooking spray. Place margarine, peanut butter and marshmallows in large microwavable bowl. Microwave uncovered on High 1 minute 30 seconds to 3 minutes, stirring after 1 minute, until mixture is melted and smooth when stirred. Stir in cereal until well coated.
2Press mixture in pan, using buttered back of spoon. Immediately make 24 indentations in rows (6x4) in mixture using handle of wooden spoon. Press peanut butter cups into indentations; cool. Cut into squares.