Once it reaches a rolling boil, stop stirring and let it boil for two minutes or until it reaches the soft ball stage (I don't use a thermometer, I just let it boil for two minutes).
Remove from the heat and quickly stir in the vanilla extract and peanut butter. Beat until smooth and quickly spread over the brownies. I let mine set a bit in the fridge before cutting and serving.
1 teaspoon vanilla extract, 1/2 cup creamy peanut butter