Preheat oven to 350F degrees. Spray a 9x13-inch baking dish with nonstick cooking spray.
In a medium bowl, combine 15 ounce can pumpkin puree, 12 ounce can evaporated milk, 3 large eggs, 1 cup sugar and 1 teaspoon ground cinnamon until well-combined. It will be very liquid-y.
Pour this pumpkin mixture into the baking dish.
Sprinkle 1 box spice cake mix evenly on top of pumpkin mixture.
Drizzle 1 cup (2 sticks) unsalted butter, melted over the top.
Bake for about 55-60 minutes.
It might still be just a bit jiggly in the middle when done. It will continue to set up as it cools.
Let it cool for at least 30 minutes before slicing and serving.
Video
Notes
Please see my Frequently Asked Questions and post above for commonly asked questions and substitutions.