Combine butterscotch chips, peanut butter and butter in a large microwave-safe dish. Microwave on 50% power for 2 minutes. Remove, stir, then microwave another 1 minute on 50% power. Cool slightly.
Line a 8x8" square baking pan with parchment paper. Add the marshmallows. When butterscotch mixture has cooled slightly, pour over marshmallows and stir to combine and spread evenly in pan. Refrigerate until firm, about 1 hour.