Whisk together the yeast, warm water and brown sugar in a small mixing bowl and let proof for 10 minutes.
Combine the brown rice flour, sweet sorghum flour, tapioca starch, corn meal, xanthan gum, cocoa powder, salt, caraway seeds and onion powder in the bowl of a stand mixer. With regular beaters (not a dough hook), mix on low speed to combine the ingredients.
Combine the boiling water with the potato buds and let sit for 30 seconds then whip with a fork. (Alternatively you can just use 1 cup of mashed potatoes). Add the potatoes to the flour mixture along with the yeast mixture, apple cider vinegar, oil and molasses. Start the mixer on low then increase speed to medium and mix for 2 minutes, scraping down the sides of the bowl with a spatula if needed.
Spray two 8- by 4-inch loaf pans well with gluten-free, non-stick cooking spray. Divide the batter evenly among the prepared pans and smooth the tops. Cover with a clean tea towel and let rise in a warm, draft free environment for an hour or until the batter is level with the top of the pan. Make 2 or 3 diagonal slashes with a sharp knife on the top of the loaves to allow steam to escape. Mix the egg white with a tablespoon of water and brush on the tops of the loaves (omit for egg free).
While the loaves are rising, preheat the oven to 375 degrees. Bake the loaves for 55 – 65 minutes or until the crust is browned and the bread makes a clicking sound when tapped with your fingernail. Do not undercook the bread or it will be too moist inside. If the tops of the loaves are browning too quickly, cover with a piece of foil.
Let cool in the pans.
Combine the cream, sour cream and salt (if using) in the bowl of a stand mixer. Start on low and gradually increase speed to medium-high. Mix until the butter starts to separate from the buttermilk, about 3 - 4 minutes. The buttermilk may splatter as it begins to separate and the butter starts to form.
While the mixture is mixing combine some ice and cold water in a large mixing bowl. Once the butter is done, gather it up into a ball in a clean tea towel, twisting the towel gently. Submerge the towel into the ice bath and swish it a few times to wash any remaining buttermilk from the surface. Unwrap the butter from the towel and place in a serving bowl. The butter is best used at room temperature. Can be stored, covered, in the refrigerator for up to 2 weeks.