Combine sugar, cocoa, butter, milk and salt in a medium sauce pan. Cook over medium-high heat until mixture comes to a rolling boil, stirring constantly. Once mixture reaches a full rolling boil cook for 1 minute, stirring constantly. Remove from heat, then add peanut butter, vanilla extract, and nuts. Beat fudge with a spoon and it will begin to thicken. When fudge begins to thicken pour into buttered 8x8 or 9x9 pan, then refrigerate to allow to set.