Preheat your oven to 350°F. Grease and line a standard 9-inch x 5-inch loaf pan and set it aside.
In a small bowl, place the flour, xanthan gum, salt, baking powder, baking soda, pumpkin pie spice, granulated sugar and light brown sugar, and whisk to combine well, breaking up any lumps.
In a separate large mixing bowl, whisk together the melted butter, eggs, sour cream, and pumpkin puree until very smooth.
Add the dry ingredients to the large bowl of wet ingredients, and mix until just combined. The batter will be very thick, but soft.
Scrape the batter into the prepared pan, pressing it into all corners of the pan so there aren’t any gaps in the baked bread.
With a wet spatula, smooth the top of the batter into an even layer and score it about 3/4-inch deep down the center of the bread using a sharp knife at a 45° angle.
Dust the top generously with a layer of the (optional) cinnamon sugar mixture, and score again down the center if necessary to reestablish the scoring.
Place the loaf pan in the center of the preheated oven and bake until the top is domed and lightly golden brown, and a toothpick inserted in the center comes out with no more than a few moist crumbs attached (nothing gooey at all!) (about 50 minutes).
Remove the pan from the oven and allow to cool for 10 minutes before turning out onto a wire rack to cool completely. Slice and serve once cool.
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Notes
To make with pumpkin butter instead of pumpkin puree.The original way I made this recipe was with pumpkin butter, which is a cooked down, sweetened version of pumpkin puree that has tons of pumpkin flavor. But since pumpkin butter takes time to make and isn't always available to buy, I now favor this version made with just pumpkin puree.To make the version with pumpkin butter, the method is the same. All of the ingredients are the same except for:
2/3 cup (5 1/3 fluid ounces) buttermilk (in place of sour cream)
5 ounces pumpkin butter (in place of pumpkin puree)
2/3 cup (133 grams) granulated sugar (instead of 1/3 cup)
1/4 cup (55 g) packed light brown sugar (instead of 1/2 cup)
6 tablespoons (84 grams) melted butter (instead of the same amount of room temperature butter)
For pumpkin pie spiceTo make your own pumpkin pie spice, combine 1 teaspoon ground cinnamon + 1/2 teaspoon ground ginger + 1/4 teaspoon ground allspice, 1/4 teaspoon ground cloves + 1/8 teaspoon ground nutmeg.For the dusting cinnamon sugarDusting the top of the raw loaf of pumpkin bread with cinnamon sugar creates a very thin, crunchy layer of fragrant sweetness. It’s entirely optional, though.I usually keep a cinnamon-sugar mixture in a small, sealed container on my kitchen counter, since it’s just so useful. I use 1 full teaspoon ground cinnamon for each 3/4 cup of granulated sugar. You should add cinnamon to taste, though.To make pumpkin swirl bread.You'll need these ingredients to make the swirl ingredients:
2 tablespoons (28 g) unsalted butter, melted
1/3 cup (73 g) packed light brown sugar
1 1/2 teaspoons pumpkin pie spice
To make the swirl, place the melted butter in a small bowl. Add the brown sugar and pumpkin pie spice, and mix to combine well.Make the batter as directed in the recipe above through step 4. Preheat your oven to 325°F.Then, transfer half of the pumpkin bread batter to the prepared loaf pan and spread it into an even layer.Top with half of the swirl mixture, and spread that into an even layer.Follow with the remaining pumpkin bread batter, spread into an even layer, and then the remaining swirl mixture, spread into an even layer.With a butter knife or offset spatula held perpendicular to the bottom of the loaf pan, swirl the batter from one short side of the pan to the other, back and forth in a looping pattern. Smooth the top of the swirled batter into an even layer.Place the pan in the center of the preheated oven at 325°F, and bake for 40 minutes. Rotate the pan and continue baking until the bread is firm when pressed gently in the center, at least another 20 minutes.If the bread still isn’t firm in the center, lower the heat to 300°F and continue baking for up to another 15 minutes.Remove the pan from the oven and allow it to cool in the pan for 10 minutes. Transfer the bread to a wire rack to cool completely before slicing thickly and serving.Nutritional information.Nutritional information is an estimate, per slice based on a loaf cut into 10 slices without the optional cinnamon sugar topping. It's estimated for the recipe as written, not any of the possible variations, using online nutritional calculators and should not be relied upon.