For the bread dough
3 cups (420 g) all purpose gluten free flour, plus more for sprinkling (I used Better Batter)
1 1/2 teaspoons xanthan gum (omit if your blend already contains it)
1/4 cup (36 g) tapioca starch/flour
1/2 cup (43 g) cultured buttermilk blend powder (or nonfat dry milk)
1 tablespoon (9 g) instant yeast
1/4 teaspoon cream of tartar
1/4 teaspoon baking soda
1 tablespoon (13 g) packed light brown sugar
1 teaspoon (6 g) kosher salt
1 teaspoon apple cider vinegar
2 tablespoons (28 g) unsalted butter, at room temperature
2 egg whites (50 g), at room temperature
1 1/2 cups + 2 tablespoons (13 fluid ounces) warm water (about 95°F)
For the filling
3 tablespoons (36 g) granulated sugar
3 tablespoons (40 g) packed light brown sugar
2 teaspoons ground cinnamon
1 tablespoon (9 g) tapioca starch/flour
2 tablespoons (28 g) unsalted butter, at very soft room temperature
For the egg wash
1 egg, any size, beaten with 1 tablespoon milk (any kind) or cream
First, make the bread dough. In the bowl of a stand mixer fitted with the paddle attachment, place the flour, xanthan gum, tapioca starch/flour, buttermilk blend, yeast, cream of tartar, baking soda, and brown sugar, and whisk to combine well, breaking up any lumps in the brown sugar. Add the salt, and whisk again to combine. Add the apple cider vinegar, butter, egg whites, and water, and mix to combine well. Turn the mixer speed up to medium-high and then high speed and beat until the dough almost seems whipped. The dough should pull away easily from the sides of the bowl when you use a silicone spatula to scrape it down.
Transfer the dough to a lightly oiled bowl or proofing bucket large enough for the dough to rise to nearly double its size. Spray the top of the dough very lightly with cooking oil spray. Cover the bowl very tightly with an oiled piece of plastic wrap or the oiled top of your proofing bucket. Place the dough in the refrigerator for at least an hour and up to 24 hours.
When you are ready to shape and bake the bread, grease an 8 1/2-inch x 4 1/2-inch loaf pan and set it aside. Place all the filling ingredients except the butter in a small bowl, mix well, and set aside. Remove the dough from the refrigerator and turn it out onto a lightly flour surface. Turn the dough over on itself, sprinkling it lightly with more flour as necessary to prevent it from sticking. Use a large rolling pin to roll the dough into a thick 7-inch x 12-inch rectangle, still sprinkling very lightly with more flour as necessary in sticky spots. Dust any excess flour off the top of the rectangle and, using a small offset spatula or a large wide knife, spread the softened butter into an even layer across the top of the dough, leaving a small clean edge (about 1/2-inch) all around. Sprinkle the cinnamon-sugar mixture evenly over the top of the butter and press down to help the mixture adhere.
Starting on a short side of the dough, roll it tightly away from your body, pressing the seam to seal as you go, and finishing the with seam side down. Lift the loaf carefully, trying to prevent any filling from falling out, and place it in the center of the prepared loaf pan. Cover the pan with a lightly greased piece of plastic wrap and place it in a warm, draft-free location to rise until about 150% of its original size.
Once the dough is nearly at the end of its rise, preheat your oven to 325°F. Once it has finished rising, remove the plastic wrap and, using a lame or small, sharp knife, slash down the center of the loaf, lengthwise, at a 45° angle and about 1/3-inch deep. Brush the top of the loaf, including inside the slash, generously with the egg wash. Place the pan in the center of the preheated oven and bake for 45 minutes. Remove the loaf from the pan, place it in a lined baking sheet, and place it back in the oven. Continue to bake until the loaf is lightly golden brown, feels firm when pressed down lightly in the center, and the internal temperature reaches about 195°F on an instant-read thermometer (about 15 minutes more). Remove the loaf from the oven and place on a wire rack to cool for at least 20 minutes before slicing and serving.