Cook first 3 ingredients in a small heavy saucepan over medium heat, stirring constantly, 5 to 6 minutes or until smooth; remove from heat. Stir in chunky peanut butter, vanilla extract, and salt until blended; stir in chopped peanuts. Pour into a buttered 9-inch square pan. Chill until firm; cut into squares. Store fudge in refrigerator.
NOTE: For ease, microwave first 3 ingredients in a 2-quart microwave-safe bowl at HIGH 2 to 3 minutes or until melted, stirring twice.