In a small saucepan over medium heat, melt butter. Whisk in the sugar and heat until sugar dissolves (about one minute). Remove from heat immediately add vanilla and allow to cool to room temperature (very important).
In a large mixing bowl, beat cream cheese with powdered sugar and peanut butter until creamy (about 3-4 minutes, don't skimp on the beating time). On low, add in brown sugar mixture (that has cooled to room temperature). Mix until combined.