Grease a standard-size loaf pan and line it with parchment paper.
Cinnamon Swirl: Mix the ½ cup granulated sugar and ground cinnamon together2 ¾ cups gluten free flour
Whisk the 2 ¾ cups gluten free flour, 2 teaspoons baking powder, ½ teaspoon baking soda, and ½ teaspoon fine sea salt together in a large bowl. In a medium bowl, whisk the 1 large egg and ¾ cup granulated sugar together until combined. Whisk in the ⅓ cup coconut oil, ⅓ cup sour cream, and ⅔ cup whole milk. Pour the wet ingredients into the dry ingredients, then whisk to completely combine.
Pour half of the batter into the prepared loaf pan and smooth it gently with a rubber spatula or offset spatula. Save about 2 tablespoons out of the ¼ cup cinnamon sugar, then sprinkle the rest evenly over the batter layer. Carefully pour and spread the remaining batter evenly on top of the cinnamon sugar. Use a knife to make a large swirl down and through the loaf pan. Sprinkle the top of the loaf with the rest of the cinnamon sugar.
Preheat oven to 350°F and rest the batter while it heats, then bake the bread for 55-65 minutes. Cover the top loosely with foil if it looks like it's over-browning. Check the bread at 55 minutes, and keep checking it until a toothpick comes out clean.
Transfer the baked loaf to a wire rack and let it cool in the pan for 1 hour, then turn out the bread to let it cool completely.
Notes
Don’t overmix. The key to a tender crumb is using a light hand when stirring. Once the dry ingredients meet the wet, stir just until combined. A few streaks of flour are better than a dense loaf.
The right swirl technique. Drag a butter knife or skewer through the batter in gentle, fluid motions. Too much swirling will blend the cinnamon in completely; too little leaves it in harsh layers. The goal is a marbled effect that seems effortless but intentional.
Letcool. The scent of cinnamon bread is nearly impossible to resist, but slicing too soon can lead to a crumb that crumbles apart, especially because this is a gluten-free quick bread. The instructions say to cool the bread for 1 hour; try to make it til at least 30 minutes so that the crumb can fully set.
Rest the batter. Like all gluten-free bakes, resting the batter for 20 minutes, or while the oven preheats, allows the flour to fully hydrate, leading to a softer crumb.