Grease a 13X9 inch baking pan; set aside. In a large bowl, stir together the brownie mix, cooking oil, eggs, and water until well mixed. Spread mixture into prepared pan.
Bake in a 350 degree oven for 28 to 30 minutes or until toothpick inserted comes out clean. Cool completely on wire rack.
Frost with frosting. Sprinkle with peanuts. Cover; chill in the refrigerator. Meanwhile, place rice cereal in a medium bowl. In a small saucepan, combine peanut butter and chocolate pieces. Heat and stir over low heat until chocolate is melted. Pour over cereal. Stir to coat evenly. Spread over frosting. Cover; chill in refrigerator until chocolate layer is set. Refrigerate leftover bars, covered.