In microwavable bowl, microwave white chocolate and butter uncovered on High about 1 minute, stirring once, until melted and chocolate can be stirred smooth. Pour over cereal mixture, stirring until evenly coated.
In large resealable food-storage plastic bag, toss half of the cereal mixture with half of the powdered sugar until evenly coated. Spread on waxed paper to cool. Repeat with remaining cereal mixture and remaining powdered sugar.
Store tightly covered at room temperature.