Spray an 8x8-inch baking pan very lightly with non-stick cooking spray. Set aside.
In a large bowl, add powdered sugar and set aside.
In a medium saucepan, melt the butter over medium heat. Once melted, stir in brown sugar and milk. Bring to a boil and then continue to boil for about 2 minutes, stirring constantly.
Remove saucepan from heat. Stir in peanut butter and vanilla. Pour peanut butter mixture over powdered sugar. Using an electric hand mixer or stand mixer with a fitted paddle attachment, beat until smooth.
Pour into prepared baking pan and refrigerate until firm, about 1 hour. Remove from refrigerator and cut into squares.