In 1-quart microwavable bowl, microwave White Chocolate Peanut Butter cups and butter uncovered on High 1 minute; stir. Microwave about 30 seconds longer or until mixture can be stirred smooth. Stir in vanilla. Pour mixture over cereal, stirring until evenly coated. Pour into 2-gallon resealable food-storage plastic bag.
Add powdered sugar. Seal bag; shake until well coated. Spread on waxed paper to cool. Add Reese’s Pieces on top (optional). Store in airtight container.