Line a 15x10x1-inch baking pan with parchment, foil or waxed paper; set aside. In a large bowl combine pretzels, oat cereal, corn square cereal, peanuts, and milk chocolate pieces; set aside.
In a microwave-safe medium bowl combine white baking pieces and shortening; microwave on 100 percent (high) power for 1 to 2 minutes or until baking pieces are melted, stirring every minute until smooth. Pour over cereal mixture; mix well.
Spread onto prepared baking pan. Cool completely; break apart. Store in an airtight container. Makes 20 servings.