In a large mixing bowl, whisk together warm water and active sourdough starter until fully dissolved. Stir in granulated sugar.

Gradually add unbleached all-purpose flour, mixing until a thick dough forms. Sprinkle in kosher salt.

Allow the dough to rest for 30 minutes, covered with a kitchen towel.

Pinch salt into the dough, kneading for several minutes until well incorporated. Shape it into a ball and let it rest for another 30 minutes.

Perform series of stretch and folds every 30 minutes for three rounds.

Cover with a damp cloth and let dough rise at room temperature for 3–4 hours, until puffy and nearly doubled.

Transfer the risen dough to a floured surface, stretch into rectangle, sprinkle with ground cinnamon and dark brown sugar, fold and roll into loaf shape.

Place dough seam side down in proofing basket and cover. Let proof in refrigerator for 2–3 hours.

Preheat oven to 450℉ with Dutch oven inside about 30 minutes before baking.

Remove hot Dutch oven, score the loaf, transfer it into the pot, cover, and bake for 25 minutes.

Uncover and bake for an additional 18–20 minutes until golden brown and hollow when tapped.

Remove from Dutch oven and cool on a wire rack for at least 2 hours before slicing.