In a large microwave-safe bowl, combine cereals and peanuts. In a small microwave-safe bowl, combine the butter, brown sugar, peanut butter and corn syrup. Microwave, uncovered, on high for 1 minute, stirring once. Pour over cereal mixture; cook on high for 3 minutes, stirring every minute.
Spread onto waxed paper to cool. Break into pieces; add chocolate chunks. Store in an air tight container.