Heat the oven to 350°F and lightly spray a 9-by-13-inch baking pan. Set the pan aside while you prepare the batter.

Add the dry spice cake mix, pumpkin puree, and water to a mixing bowl. Add the optional pumpkin-pie spice if you want a more pronounced pumpkin flavor, then stir only until the ingredients are evenly blended.

Spread the thick batter evenly in the prepared pan. Use the back of a spoon or a small offset spatula to guide the batter into the corners and smooth the surface.

Bake for approximately 25 to 30 minutes, or until the center is set and a toothpick inserted in the middle comes out with only a few moist crumbs. Avoid adding extra baking time unless the center still looks wet.

Cool the bars in the pan for 15 to 20 minutes before cutting them into 24 squares. For especially neat pieces, let them cool completely before slicing with a sharp knife.