Preheat the oven to 350 degrees F and grease a 9 by 13 inch baking dish well.
In a large bowl cream the butter and sugar together well.
Add the pumpkin and beat well to combine.
Add the eggs, one at a time, mixing well after each addition.
Add the evaporated milk, salt, and pumpkin pie spice and mix well.
Add the yellow cake mix and mix well to combine.
Add the batter to the prepared pan and cook until a toothpick inserted into the center comes out clean and the cake springs back when lightly touched, about 45 minutes.
Serve warm or at room temperature with a dusting of powdered sugar and fresh whipped cream.
Notes
This cake tastes great if made the day before. Store it in the refrigerator and bring it to room temperature before serving. It's a great candidate for a dessert to make ahead of time and it travels really well too.