Line a 9-in. square pan with foil and grease the foil with 1 teaspoon butter; set aside., Cube remaining butter and place in a large heavy saucepan. Add the sugar, milk and salt; cook and stir over medium heat until sugar is dissolved. Bring to a rapid boil; boil for 5 minutes or until a candy thermometer reads 230°, stirring constantly. , Remove from the heat; stir in chips until melted. Stir in marshmallow creme, peanut butter and vanilla until blended. Pour into prepared pan; refrigerate for 2 hours or until firm., Using foil, lift fudge out of pan. Gently peel off foil; cut fudge into 1-in. squares. Store in an airtight container in the refrigerator.