Combine the Ingredients
Start by mixing the flour, sugar, and salt in a large mixing bowl. Add the warm water and the sourdough starter, and stir until a rough dough forms. Add in the softened butter, mixing until it’s fully incorporated. The dough will be soft and a little sticky—that’s exactly what you want.

Let the Dough Rest and Stretch
Once mixed, cover the bowl with a clean towel and let the dough rest for 30 minutes. After resting, perform a set of gentle “stretch and folds.” To do this, grab one edge of the dough, stretch it upward, then fold it over itself. Do this 3–4 times around the bowl. Repeat this resting and stretching process two more times every 30 minutes.

Bulk Fermentation
After the final fold, cover the bowl and let the dough rise at room temperature until it roughly doubles in size. This could take anywhere from 6 to 8 hours, depending on the temperature of your kitchen and the strength of your sourdough starter.

Shape and Pan the Dough
Once the dough has doubled, turn it out onto a lightly floured surface. Shape it gently into a log shape and transfer it into a greased 9×5-inch loaf pan. Don’t press it down—just nestle it in gently.

Final Rise
Cover the pan with a light cloth or plastic wrap and let the dough rise again until it rises just above the rim of the loaf pan. This might take 1–2 hours. Be patient here—the final rise gives the bread its beautiful shape and soft interior.

Bake
Preheat your oven to 375°F (190°C). Bake the loaf for 35–40 minutes, or until the crust is a rich golden brown. It should sound hollow when tapped on the bottom. If the top starts browning too quickly, you can lightly cover it with foil halfway through baking.

Cool Completely
Remove the bread from the pan and let it cool on a wire rack. Give it at least 1 hour before slicing. This helps set the crumb and keeps the inside from becoming gummy.