In a large mixing bowl, whisk together pumpkin and sugars until well combined. Whisk in eggs, oil, and vanilla until smooth.
15 ounces pumpkin purée, 1 1/2 cups granulated sugar, 1/2 cup brown sugar, 3 large eggs, 1 cup vegetable oil, 1 teaspoon pure vanilla extract
Add the dry ingredients to the pumpkin mixture and stir well using a rubber spatula. Make sure to scrape the bottom and sides of the bowl while mixing, but do not overmix the batter; only stir until all of the dry bits are incorporated.
Divide the batter between the prepared pans and smooth the tops.
For the optional topping: Use a knife to cut a shallow trench down the middle of each loaf. Pour half of the melted butter into each trench. Sprinkle 1 teaspoon cinnamon sugar over the top of each loaf.
1 tablespoon butter, 2 teaspoons cinnamon sugar
Place on the center rack and bake for about 60 minutes, until a toothpick inserted into the center comes out clean. Let cool in the pans for 10 minutes, then remove and cool completely on a wire rack before cutting.
Notes
9x5-inch pans can be used, but the loaves will be squatter. Start checking for doneness around the 50-minute mark.
The individual spices can be swapped for 1 tablespoon pumpkin pie spice.