Line 13x9-inch pan with foil, leaving foil overhanging at 2 opposite sides of pan; grease foil generously with shortening or butter. In 3-quart saucepan, melt 1/4 cup butter over medium heat. Add marshmallows; cook 2 to 3 minutes, stirring frequently, until melted. Remove from heat; stir in peanut butter until smooth. Stir in cereal.