1. Heat 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.)
2. Make and bake brownies as directed on box for 13x9-inch pan, using water, oil and eggs. Cool completely, about 1 hour.
3. Frost brownies with frosting. Sprinkle with peanuts; refrigerate while making cereal mixture.
4. Measure 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. Cool completely before cutting, about 1 hour. For bars, cut into 6 rows by 4 rows. Store tightly covered at room temperature.