Line 13x9-inch pan with foil, with ends of foil extending over sides of pan. Lightly grease foil; set aside.
Place butter and peanut butter in large saucepan; cook on low heat until butter and peanut butter are completely melted and mixture is well blended, stirring frequently.
Add marshmallows; cook until melted, stirring constantly. Remove from heat. Stir in cereal. Press firmly into prepared pan; cool completely.
Lift dessert from pan, using foil handles. Cut into 24 bars. Store in tightly covered container at room temperature.