Gather all ingredients and measure them out. Line a 9x13 cookie sheet with heavy duty foil, set aside.
Combine first 3 ingredients in a large (5 qt.) pan. Bring to a full rolling boil, stirring constantly. Cook for 6 minutes or to 234 degrees on a candy thermometer. Carefully add vanilla-substance (will bubble when added). Remove from heat. Stir in peanut butter chips and marshmallow cream until smooth. Add chopped nuts (opt).
Pour mixture into prepared cookie sheet. Let cool and firm. When cooled remove from pan with foil. Wrap foil around fudge to seal, will keep in a cool dry place for 7-10 days (if your family does not find it). Slice into bite size pieces and enjoy. Keep covered.