Step 1

Prepare cake mix in a 9x13 baking dish as directed on the package. Allow it to cool.

Step 2

In a large saucepot, combine sugar, milk, shortening, butter, and salt. Bring to a rolling boil over medium-high heat, stirring constantly to prevent scorching.

Step 3

Once it reaches a boil, let it boil for two to three minutes without stirring.

Step 4

Remove from heat and immediately stir in vanilla and peanut butter. Beat until icing is smooth and starts to lose its shine. Immediately pour it onto the cake and allow it to cool before serving.