LIne a 9x13 inch pan with parchment or waxed paper.
In a microwave safe bowl, melt butterscotch chips in the microwave or over a double boiler, stirring every 30 seconds until smooth. Stir in the melted butter and peanut butter. Measure the marshmallows and rice cereal into a large bowl. Pour the butterscotch mixture over the marshmallows and cereal. Stir to coat evenly. Press onto the prepared pan.
Chill for 20 to 30 minutes, until set. Cut into bars. Store leftovers in the refrigerator.