1Spray bottom and sides of 13x9-inch pan with cooking spray. In large bowl, place cereal and marshmallows; stir to mix. Set aside.
2In medium microwavable bowl, microwave peanut butter chips, corn syrup and butter uncovered on High 2 to 3 minutes, stirring after each minute, until mixture is smooth. Remove from microwave.
3Pour peanut butter mixture over cereal mixture in bowl; stir until cereal is evenly coated. Pour cereal mixture into pan. With buttered back of spoon, press mixture in pan until even. Spread melted chocolate evenly over top. Refrigerate about 1 hour or until firm.
4With table knife, cut cereal mixture into 9 rows by 4 rows to make 36 bars. Store in loosely covered container.