Peanut butter icing

This made an easy, frugal icing for our cake!  Yum!  Another recipe from Southern Plate.

1 box yellow cake mix
1 1/2 Cups sugar
7 Tbsp milk
2 Tbsp shortening
2 Tbsp margarine(I used butter)
1/4 tsp salt
1 tsp vanilla extract
1/2 C creamy peanut butter

Prepare the cake mix according to the package directions, baking it in a 9x13 inch pan.

While the cake is cooling, combine the sugar, milk, shortening, margarine, and salt in a medium heavy saucepan. Bring to a rolling boil, stirring constantly to prevent scorching.  Once it reaches a rolling boil, stop stirring and let boil for 1 to 2 minutes or until it reaches the soft-ball stage.  Remove from the heat and quickly stir in the vanilla and peanut butter.  Beat until smooth and quickly spread onto the cake.