Prepare a 9-inch square pan with parchment paper.
In large Dutch oven over low heat, melt marshmallows with butterscotch chips and butter; stir occasionally until smooth. Add popcorn; toss until well coated.
Press half of mixture firmly into the 9-inch square pan. Spread fudge sauce evenly over mixture. Press remaining popcorn mixture over fudge. Allow mixture to cool and harden. You can also refrigerate it.
Remove entire mixture and slide into cutting board to cut into bars.