1. Place a saucepan over medium heat, and melt butter in it.
2. Add milk, and brown sugar. Give it a good stir. Boil for 2 minutes.
3. Remove from heat, and drop vanilla and peanut butter in it. Mix well.
4. In a large mixing bowl, add icing sugar, and pour hot fudge over it. Beat with mixer until nice and smooth.
5. Line a greased pan with parchment paper. Pour mixture, and spread evenly.
6. Pop it in refrigerator for at least 1 hour or until nice and firm.