Heat the oven to 350°F (175°C). Lightly grease a 9 x 13-inch baking dish so the pumpkin cake can be served in neat squares after cooling.

In a large bowl, stir together the pure pumpkin purée and evaporated milk until the mixture looks smooth and evenly blended.

Add the dry spice cake mix and stir just until the dry ingredients disappear. The batter will be thick, so avoid mixing longer than necessary.

Spread the batter evenly in the prepared baking dish. Use the back of a spoon or a spatula to reach the corners and create an even layer.

Bake for approximately 40–50 minutes, beginning to check around 40 minutes. The center should be set, and a toothpick inserted near the center should come out with a few moist crumbs.

Cool the cake for at least 15–20 minutes before slicing. Serve plain, or add whipped cream or vanilla ice cream just before serving.