In the bowl of a stand mixer fitted with the paddle attachment, mix the flour, xanthan gum, cream of tartar, sugar, salt and yeast to combine. Add Sourdough Starter and butter and mix to combine.
With the mixer on low, pour in the milk in a slow, steady stream. Once the flour has begun to incorporate the liquids, beat the ingredients on at least medium speed for 4 to 6 minutes. The dough will be pretty sticky—thicker than cake batter, not quite as thick as cookie dough. Scrape the dough into the greased loaf pan and smooth the top with wet hands.
Allow the dough to rise in a warm, humid place for 30 to 45 minutes or until it has about doubled in size. (In a colder, drier environment, this will take longer. If the environment is warm and humid, it may take less time.) While the dough is rising, preheat the oven to 400°F.
Bake the loaf in preheated oven for 40 to 45 minutes or until a nice, golden brown crust has formed on top.