Melt butter in medium saucepan over medium heat. Stir in brown sugar and milk.Bring to a boil and boil for two minutes stirring frequently. Remove from heat.
Stir in peanut butter and vanilla.pour over powdered sugar in a large mixing bowl. Beat until smooth. Pour into a 9x9 dish that has been prepared by lining the dish with foil, extending foil over the edges. Spray lightly with non stick cooking spray. with a piece of foil that extends over the edge of dish. Chill until firm. Remove fudge from dish using the extended foil to take out of dish and then remove foil from fudge and cut into squares.