Combine butter, chocolate and butterscotch chips in 1-quart measure.
Microwave 2 to 3 minutes on HIGH, or until chips have softened and can be stirred easily.
Beat with a fork until well blended.
Mix marshmallows and nuts in 2-quart casserole.
Pour in melted mixture.
Drop by spoonfuls onto waxed paper.
Refrigerate until firm.