1. Combine Wet Ingredients: In a large mixing bowl, gently mix the active sourdough starter with the warm water until the starter is mostly dissolved. This creates a milky, slightly bubbly liquid.

2. Add Dry Ingredients: To the wet mixture, add the salt, bread flour, and unsweetened cocoa powder. Mix with a sturdy spoon or your hands until a shaggy dough forms and no dry streaks of flour remain. Cover the bowl with a damp cloth or plastic wrap and let it rest for 30 minutes (this is called autolyse, allowing the flour to fully hydrate).

3. Perform Stretch and Folds: After the rest, perform your first set of stretch and folds. With wet hands, grab a portion of the dough from one side, stretch it upwards, and fold it over to the opposite side of the bowl. Rotate the bowl a quarter turn and repeat this process 3-4 more times until the dough feels slightly stronger. Cover and let it rest for 30 minutes.

4. Incorporate Chocolate and Walnuts: After the second stretch and fold (or even the third, if you prefer), gently flatten the dough slightly. Sprinkle the chocolate chips and chopped walnuts evenly over the surface. Gently fold the dough over itself a few times to evenly distribute the add-ins, being careful not to overwork the dough. Continue with 2-3 more sets of stretch and folds, spaced 30 minutes apart, until the dough feels elastic and holds its shape better.

5. Bulk Fermentation: After the stretch and folds, allow the dough to bulk ferment at room temperature for 4-6 hours, or until it has increased in volume by about 30-50% and feels airy and bubbly. The exact time will depend on your room temperature and starter activity.

6. Shape and Cold Proof: Gently turn the dough out onto a lightly floured surface. Shape it into a round or oval loaf, creating tension on the surface. Place the shaped dough seam-side up into a floured banneton or a bowl lined with a floured kitchen towel. Cover and refrigerate for 8-12 hours, or up to 24 hours. This cold proof develops flavor and makes the dough easier to handle.

7. Preheat and Bake: Preheat your oven to 450 F (230 C) with a Dutch oven inside for at least 30 minutes. Carefully remove the hot Dutch oven, invert the cold dough into it, and score the top with a sharp knife or lame. Bake covered for 20 minutes, then remove the lid and continue baking for another 20-30 minutes, or until the crust is deep golden brown and the internal temperature reaches 200-210 F (93-99 C).

8. Cool Completely: Transfer the baked loaf to a wire rack and let it cool completely for at least 2-3 hours before slicing. This is crucial for the crumb to set and for the flavors to fully develop.