Peanut Butter-Filled Chocolate Cereal Bars

Spray 13x9-inch pan with cooking spray. In large microwavable bowl, microwave 1/4 cup butter uncovered on High 30 to 45 seconds or until melted. Add marshmallows; toss until coated. Microwave uncovered on High 1 minute 15 seconds to 1 minute 45 seconds, stirring every 30 seconds, until mixture can be stirred smooth. Stir in cereals. Press evenly in pan. In medium bowl, beat filling ingredients with electric mixer on medium speed until smooth. Spread filling evenly over base. In small microwavable bowl, microwave chocolate chips and 1/4 cup butter uncovered on High 30 to 60 seconds or until mixture can be stirred smooth. Spread over filling. Sprinkle with peanut butter pieces. Refrigerate about 30 minutes or until set. For bars, cut into 6 rows by 4 rows. Store loosely covered.