Mix the dough and autolyze. Combine the flour, brown sugar, buttermilk, melted butter (or oil), and sourdough starter in a large mixing bowl. Then, stir until a sticky dough forms. Cover the bowl with a clean tea towel and rest on the counter for 30 minutes. This rest period, autolyze, allows the flour to hydrate fully and helps improve the dough’s texture.
Knead briefly and add salt. After the dough has rested, sprinkle the salt over it and knead by hand for 1-2 minutes directly in the bowl or on a lightly floured counter. The dough will still feel sticky, but it should begin to smooth out as you knead. Make sure the salt is fully incorporated and the dough no longer feels gritty.
First rise. Cover the dough with a damp towel and allow it to rise warmly for 4 hours or until it has doubled in size. During this time, gently fold the dough back into a ball once or twice (about every hour or two) to help build the structure. If the towel dries out, re-dampen it to keep the dough from forming a skin.
Shape the dough. Turn the dough onto a clean surface and gently stretch it into a rectangle, approximately 9 inches by 18 inches. Then, fold the corners inward and roll the dough tightly into a log. Next, tuck the ends under and pinch the seams along the side and bottom to seal the loaf. Transfer the dough to a greased loaf pan, seam side down. Cover the loaf with greased plastic wrap.
Second rise. Let the dough rise again in a warm place for 3-4 hours or until it rises 1-2 inches above the edge of the loaf pan, forming a dome shape. Towards the end of this rising time, preheat your oven to 350°F with the rack in the center.
Bake the bread. Once the loaf has risen, carefully remove the plastic wrap. Brush the top of the dough with 1 tablespoon of melted butter. Bake for 35-45 minutes until the bread is golden brown, and the internal temperature reaches 200°F in the center. If the top starts to brown too quickly, loosely cover it with foil.
Cool and slice. Remove the bread from the loaf pan and allow it to cool on a wire rack. Wait until the bread has cooled completely for the neatest slices before slicing.