LINE 13 x 9-inch baking pan with foil.
MICROWAVE semi-sweet morsels, butterscotch morsels and peanut butter in large, uncovered, microwave-safe bowl on MEDIUM-HIGH (70%) power for 2 minutes; STIR. The morsels may retain some of their original shape. If necessary, microwave at additional 10- to 15-second intervals, stirring until melted. Cool for 1 minute. Stir in marshmallows and peanuts.
SPREAD into prepared pan. Refrigerate until firm. Cut into squares.