Recipe courtesy of Culinarily Obsessed.
1/2 cup butter
2 1/4 cups brown sugar
1/2 cup milk
3/4 cup peanut butter
1 teaspoon vanilla extract
3 1/2 cups confectioners' sugar

In medium saucepan, melt butter over medium heat. Stir in brown sugar and milk. Bring mixture to a boil and boil for 2 minutes. Remove from heat and stir in peanut butter and vanilla. Pour over confectioners sugar in a large mixing bowl. Beat until smooth; pour into an 8x8 inch dish lined with tinfoil and buttered. Chill until firm and cut into squares.