Line a 9x9 pan with foil, leaving the ends of the foil extended over the sides (so you can easily pick it up out of the tray once it sets). Spray foil with cooking spray. There's nothing sadder than your fudge sticking to the foil!
Bring sugar, butter and milk to a full rolling boil over medium heat, stirring constantly.
Reduce heat and simmer until a candy thermometer reaches 234°F, stirring constantly. Keep the candy thermometer in the middle of the pan and don't allow the tip to touch the bottom or sides of the pan for a proper reading. This usually takes around 5 minutes!
Remove from heat and mix in peanut butter.
Add marshmallow creme and vanilla. Beat until well blended.
Pour into the prepared pan and allow to cool. You can leave it on your counter or stick it in the fridge to help speed it along!
Remove foil from the pan, remove the fudge from the foil (unless you want bits of foil in your fudge!), and cut into bite-sized pieces.
Enjoy! Good luck not eating it all in one sitting.