Creamy Peanut Butter Fudge

1⅓ ; cups milk3#8531; cups sugar1 teaspoon salt4 tablespoons unsalted butter1 cup creamy peanut butter2 teaspoons vanilla extractIn heavy-bottomed, large pot, combine milk, sugar and salt. Over medium heat, with a wooden spoon, stir until sugar is dissolved. Continue to heat mixture without stirring until it reaches 240 degrees on a candy thermometer. Do not go over 240 degrees.While mixture heats, line an 8-inch square pan with aluminum foil and lightly coat with butter. When mixture is hot enough, remove pot from stove, add the butter, peanut butter and vanilla. Let rest without stirring 5 minutes.Quickly combine all ingredients with electric mixer until just barely combined. Pour into pan. The fudge will harden rapidly; do not overmix. Cut into 1-inch pieces.Makes 64 pieces.