Preheat the oven to 350 degrees F. Lightly great the bottom only of a 9x13 baking dish.
In a large bowl, beat together the cake mix, pumpkin puree, eggs, and water on low speed about 30 seconds until combined. Beat on medium speed for 2 minutes.
Pour into prepared baking pan and bake for approximately 30 minutes until top springs back when touched and toothpick comes out completely clean. Do not underbake; the pumpkin keeps this cake very moist so a few minutes over is better than under.
Allow cake to cool completely before frosting.
To make frosting: beat together the softened cream cheese and butter until smooth. Beat in the vanilla and salt. Gradually beat in the powdered sugar, adding 1-2 tablespoons milk as needed so you can incorporate all the sugar.
Frost cake and serve. Store leftovers covered at room temperature.