Butter a 8"x8"x2" baking pan. (For less thick fudge, use a small rectangular pan.) In a 2 quart saucepan, mix brown sugar, evaporated milk, and salt. Stirring over low heat, bring to a boil and boil for 5 minutes. Remove from heat. Stir in chocolate chips, peanut butter, and vanilla until chips and melted and mixture is smooth. Pour into the pan. Refrigerate at least 4 hours or until firm. Cut into small squares.