Prepare cake mix in a 9x13 pan as directed on package. Allow to cool.
In a large sauce pot, combine sugar, milk, shortening, butter, and salt. Bring to a rolling boil over medium high heat, stirring constantly to prevent scorching.
Once it reaches a boil, let boil for two to three minutes without stirring.
Remove from heat and immediately stir in vanilla and peanut butter. Beat until icing is smooth and starts to lose it's shine. Immediately pour onto cake. Allow to cool before serving.