Combine the yeast, 1 tablespoon of sugar and warm water in a small mixing bowl and whisk to combine. Let sit until the mixture is foamy, about 6 minutes.
In the bowl of a mixer whisk together the brown rice flour, tapioca starch, remaining 4 tablespoons of sugar, xanthan gum and salt. Make a well in the center and add the yeast mixture. Put the bowl in the mixer and mix on low speed to combine. Add 3 of the eggs, one at a time, mixing each egg in thoroughly. Add 8 tablespoons of butter, one tablespoon at a time, mixing after each addition until the butter is fully incorporated. Increase the speed of the mixer to medium and beat for another minute or two until the dough is smooth. Scape the dough into a clean mixing bowl, cover with a tea towel and let sit in a warm, draft free place until it has doubled in size, about 2 hours. If it looks a bit more than doubled, that’s ok.
Butter an 8- by 4-inch loaf pan with the remaining tablespoon of butter. Make sure you really coat the pan well. Scrape the batter into the prepared pan and smooth into the pan evenly. With a wet spatula, smooth the top. Cover with a tea towel and let rise for 45 minutes. (To do a cool rise in the fridge, cover the pan with a tea towel and let rise in the refrigerator from 2 to 24 hours. When ready to bake, uncover the pans and let the pans sit at room temperature for 15 minutes while the oven preheats.)
Place the rack of the oven in the bottom third and preheat to 350 degrees.
Cut a slit about ¼ inch deep down the center of the loaf with a sharp knife or razor blade dipped into some tapioca starch. Mix the remaining egg with 1 tablespoon of water, whisking well. Brush the mixture onto the top of the loaf, let sit for 5 minutes then brush again with the egg wash. Bake for 40 – 45 minutes or until the bread is deeply golden brown and the bread sounds a little hollow when tapped. Let cool in the pan for 15 – 20 minutes.
The bread stays moist for several days. Wrap well in plastic wrap and store in the fridge.
Nutrition facts were calculated for the entire loaf. Serving size varies depending on how large you cut the slices; please divide the amounts according to your preference.