Line a 13-in. x 9-in. pan with foil and butter the foil with 1-1/2 teaspoons butter; set aside. In a large saucepan, combine the sugar, marshmallows, milk, salt and remaining butter. Bring to a boil over medium heat, stirring constantly. Cook and stir until smooth.
Remove from the heat. Stir in the chips, peanut butter and vanilla until smooth. Fold in peanuts. Spread into prepared pan. Refrigerate for 2 hours or until set. Using foil, lift fudge out of pan; cut into 1-in. squares. Store in an airtight container in the refrigerator.