Prepare an 8" pan by lining with foil and coating with butter. Set aside.
Melt butter in a heavy saucepan. Add brown sugar and milk. Bring to a boil and boil for exactly 2 minutes, stirring constantly. Remove from heat and add peanut butter and vanilla.
Measure powdered sugar into a large bowl and pour hot mixture over the top. Beat till smooth. Stir in chopped peanuts if desired.
Pour mixture into prepared pan. Chill till set. Cut into small squares.