Butter a large plate and set aside. In a heavy saucepan, bring the sugar, buttermilk, and salt to a boil over medium-high heat. Cook to the soft ball stage, removing from the heat immediately when the candy thermometer reaches 234 degrees. Add the vanilla and peanut butter, and stir with a wooden spoon to blend. Then beat energetically with the spoon until the fudge begins to lose its gloss and starts to thicken. Pour quickly onto the plate. Allow to cool to room temperature before cutting.