Spray 13 x 9-inch pan with cooking spray. In very large microwavable bowl, microwave butter, chocolate chips and peanut butter uncovered on High 1 minute. Stir; microwave about 30 seconds longer or until mixture can be stirred smooth.
Add marshmallows; toss until coated. Microwave uncovered on High 1 minute 15 seconds to 1 minute 45 seconds, stirring until mixture is well blended.
Immediately stir in cereal gently until evenly coated. Press cereal mixture evenly into pan using spatula sprayed with cooking spray. Sprinkle lightly with powdered sugar. If desired, refrigerate about 20 minutes, or cool completely at room temperature until bars are set. For bars, cut into 8 rows by 4 rows. Store tightly covered.