1Grease bottom and sides of 9-inch square pan with shortening, or spray with cooking spray.
2In 3-quart saucepan, heat peanut butter, butter and marshmallows, stirring constantly, until melted; remove from heat. Gently stir in cereal, 1 cup at a time, until evenly coated.
3Press mixture evenly in pan with buttered spatula. Cool completely, about 30 minutes. For bars, cut into 6 rows by 3 rows.