Preheat the oven to 325°F. Lightly grease a 9 X 13 baking pan and set it aside.
Using an electric mixer, cream the butter and peanut butter until well incorporated. Add eggs, cake mix, and water. Beat on medium for two minutes. Pour the batter into the greased baking dish.
Bake for approximately 40 minutes or until a toothpick inserted into the center comes out clean. Allow the cake to cool completely before frosting.
In a mixing bowl, cream together the butter and peanut butter until well incorporated.
Add confectioners’ sugar, vanilla extract, salt, and 2 tablespoons of milk. Continue mixing until the frosting achieves a nice and fluffy consistency.
If the frosting is too thick, add an additional tablespoon (or more) of milk until the desired consistency is reached.