Line an 8-inch square baking pan with heavy-duty aluminum foil, making sure it is tucked into all corners and there is at least 1 inch overhanging top on all sides.
In a medium saucepan, bring 2 inches of water to a bare simmer. Place butter, chocolate, salt and condensed milk in a stainless-steel bowl big enough to rest on top of saucepan and set it over pan, making sure it doesn't touch water. Heat, whisking occasionally, until chocolate has melted. Don't overheat.
Remove bowl from pan and mix in popcorn and peanuts until fully combined. Scrape into prepared pan and refrigerate until set, at least 3 hours. Cut fudge and serve or store in refrigerator in an airtight container for up to 1 week.