Line a 13x9x-2-inch baking pan with foil, extending foil over edges of pan. Lightly butter the foil; set aside.
In a large, heavy-bottomed saucepan, add chocolate and butterscotch-flavored pieces or peanut butter-flavored pieces. Heat and stir mixture constantly with a wooden spoon over very low heat until chocolate is melted and smooth.
Remove saucepan from heat. (If you use a microwave, cook chocolate and the butterscotch-flavored pieces, uncovered, in microwave-safe bowl until just soft. Stir often to keep heat evenly distributed.) Stir in the peanut butter. Stir in marshmallows and peanuts.
Spread mixture into prepared pan. Chill until firm. When firm, use foil to lift it out of pan. Cut bars into squares; store in a tightly covered container in refrigerator up to 3 days.