Mix together the peanut butter and marshmallow cream in a bowl; set aside. Lightly grease a square glass dish with butter.
Stir together the sugar and milk in a saucepan over medium-high heat; stirring occasionally, bring to a boil at 234 degrees F (110 degrees C) and then immediately remove from heat and stir in the peanut butter mixture and the vanilla; pour into the prepared dish and allow to cool to room temperature before serving.
