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.
Notes
For the flour blend You can use any of my recommend all purpose gluten free flour blends with a superfinely ground rice flour as a base. I like Nicole's Best multipurpose with added xanthan gum, Better Batter's classic blend, Vitacost multi gf blend with added xanthan gum, and Bob's Red Mill 1-to-1 with an extra 1/2 teaspoon xanthan gum added because it doesn't have enough. For pumpkin pie spice To 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 sugar Dusting 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 use 1 full teaspoon ground cinnamon for each 3/4 cup of granulated sugar. You should add cinnamon to taste, though.Nutritional information. Nutritional information is an estimate, per slice based on a loaf cut into 10 slices without the optional cinnamon sugar topping.