Combine butterscotch chips, peanut butter and butter in a large saucepan and heat on low. Stir constantly until chips are almost melted. Remove from heat and stir until smooth.
Let stand for about 15 minutes, until mixture is lukewarm.
Add marshmallows and stir until coated.
Grease a piece of wax paper and use it to press the mixture firmly into prepared pan. Chill until firm. Cut and serve.
Notes
To save time: microwave the butter, peanut butter and butterscotch chips in 30 second increments, stirring after each time. Once chips are almost melted, remove from microwave and stir until smooth. Cool to lukewarm and add marshmallows.