In large bowl, mix cereals, pretzels, Reece's Pieces and peanuts. Set aside.
In microwaveable bowl, microwave white chocolate and butter on high for about 1 minute, stirring once, until melted and chocolate can be stirred smooth. Pour over cereal mixture, stirring until coated evenly.
In large, resealable food storage bags, toss half of the cereal mixture with 1/4 cup of the powdered sugar. Spread on wax paper to cool. Repeat with the remaining cereal mixture and powdered sugar.
Store tightly covered.
Makes about 20 servings, so it is great for gifts!