Preheat the oven to 325°F. Grease an 8.5 by 4.5 inch loaf pan with butter or cooking spray.
In a medium bowl, whisk the 1½ cups sugar, 15 ounces pure pumpkin, ½ cup melted butter, 2 large eggs, and 1 teaspoon vanilla extract together until smooth.
In another bowl, whisk together the 1¾ cups all purpose flour, 1 teaspoon baking soda, ½ teaspoon baking powder, ½ teaspoon salt, 2 teaspoons pumpkin spice, and 1 teaspoon ground cinnamon.
Add the dry ingredients to the wet ingredients and stir just until the flour disappears. The batter will look lumpy and that's correct. Do not overmix.
Pour the batter into the prepared loaf pan and smooth the top.
Transfer the loaf pan to the oven and bake for 70 to 80 minutes, starting to check at 65 minutes. It's done when an instant read thermometer inserted into the center reads 200 to 205°F, the top is domed and cracked, and the edges have pulled slightly away from the pan. If the top browns before the center sets, tent it loosely with foil.
Cool in the pan for 10 minutes, then turn out onto a wire rack. Let it cool completely, at least 2 hours, before slicing.