LINE 13x9-inch pan with foil; spray with cooking spray. Set aside. Microwave brown sugar, corn syrup, 2 Tbsp. peanut butter and butter in large microwaveable bowl on HIGH 2 min. or just until peanut butter and butter are melted and mixture is well blended, stirring after each minute. Add cereal; mix well. Press firmly into prepared pan.
MICROWAVE chocolate and 1/3 cup peanut butter in microwaveable bowl on HIGH 1 to 2 min. or until chocolate is completely melted when stirred, stirring after each minute. Spread over cereal layer in pan; sprinkle with peanuts.
REFRIGERATE 1 hour or until firm. Cut into 16 bars; cut each bar diagonally in half to make two triangles. Store in airtight container in refrigerator up to 4 days.