Grease an 8 inch square pan and line with wax paper or foil being sure to line the sides of the pan. Spray wax paper with an even and light coating of pan spray. Set aside. In a medium saucepan over medium heat, melt butter.
Stir in brown sugar and milk. Bring to a simmer, stirring constantly until butter is incorporated (about 1 minute).
Remove from heat and whisk peanut butter, vanilla and salt.
Place confectioners sugar in a mixing bowl attached to a stand mixer. Pour peanut butter mixture into bowl and beat on medium speed until smooth.
Pour into pan and chill until firm. Invert onto a cutting board and cut into 2 inch squares squares.