1Heat oven to 350°F (325°F for dark or nonstick pan). Grease bottom only of 13x9-inch pan with shortening or cooking spray. (For easier cutting, line pan with foil, then grease bottom only of foil.) Make brownies as directed on box for 13x9-inch pan. Cool completely, about 1 hour.
2Frost brownies with frosting. Sprinkle with peanuts; refrigerate while making cereal mixture.
3Measure cereal into large bowl; set aside. In 1-quart saucepan, melt peanut butter and chocolate chips over low heat, stirring constantly. Pour over cereal in bowl, stirring until evenly coated. Spread over frosted brownies. Refrigerate about 1 hour or until set before cutting. For bars, cut into 6 rows by 4 rows. Store tightly covered in refrigerator.