Line a 9-in. square pan with foil and grease the foil with 1 tablespoon butter; set aside. In a heavy saucepan, combine the sugar, evaporated milk, salt and 4 tablespoons butter. Cook and stir over medium heat until mixture comes to a rapid boil. Boil for 8 minutes or until a candy thermometer reads 226°, stirring constantly. Remove from the heat. Add peanut butter chips; stir until mixture is smooth. Pour into prepared pan. Refrigerate for 30 minutes.
Meanwhile, in a heavy saucepan, combine brown sugar, corn syrup and remaining butter. Cook and stir until smooth. Bring to a boil over medium heat. Remove from the heat. Stir in confectioners' sugar until smooth.
Spread evenly over peanut butter layer in pan. Sprinkle with walnuts; press down lightly. Refrigerate for 2 hours or until firm. Using foil, lift fudge out of pan. Cut into 1-in. squares. Store in an airtight container in the refrigerator.