Microwave chocolate morsels in a large glass bowl on high for 2 minutes or until they're fully melted, stirring every 30 seconds. Add peanut butter, stirring until mixture is well blended.
Spoon in cereal, peanuts and marshmallows. Line a 13-by-9-inch pan with foil; lightly grease foil. Press mixture into pan. Drizzle with melted chocolate. Sprinkle with white nonpareils, if desired.