Lightly spray a 8x8 dish with non-stick cooking spray; set aside.
In a medium pan stir together the sugar and the milk. Over medium-low heat, bring to a boil. Continue to boil for 2 ½ minutes, stirring continuously to prevent sticking.
Remove from the heat, stir in the peanut butter and the vanilla until smooth.
Pour into the prepared pan and allow to cool completely.