Sift powdered sugar and add to a large mixing bowl, set aside. In a medium saucepan over medium heat, melt butter.
Stir in brown sugar and milk.
As soon as bubbles start showing up, set a timer for 2 minutes and continue boiling.
Remove from heat after 2 minutes and stir in peanut butter and vanilla.
Pour peanut butter mixture over powdered sugar and beat until smooth. Pour into a parchment paper lined 8 x 8" baking dish.
Chill until fudge is set. Cut into 2 x 2 squares.