1Heat oven to 350°F. Grease bottom only of 13x9-inch pan with cooking spray or shortening. (For easier cutting, line pan with foil, then grease foil on bottom only of pan.)
2Make and bake brownies as directed on box. Cool completely.
3Frost brownies with frosting. Sprinkle with peanuts; refrigerate while making cereal mixture.
4Measure 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 1 hour or until set before cutting. For bars, cut into 6 rows by 4 rows. Store tightly covered at room temperature or in refrigerator.