1. prepare the sourdough base: in a large mixing bowl, combine the active sourdough starter, warm water, 1 tablespoon of sugar, and salt. stir until the starter is mostly dissolved. gradually add the 5 cups of bread flour, mixing with a wooden spoon or your hands until a shaggy dough forms. cover the bowl with a damp cloth or plastic wrap and let it rest for 30 minutes (autolyse). this step helps the flour fully hydrate.

2. knead and bulk ferment: after the rest, transfer the dough to a lightly floured surface. knead for 5-7 minutes until it becomes smooth and elastic. it should pass the windowpane test (stretch a small piece of dough thin enough to see light through it without tearing). place the dough back in the clean bowl, cover, and let it bulk ferment at room temperature (around 70-75F) for 4-6 hours, or until it has nearly doubled in size. alternatively, you can refrigerate it overnight for a slower, more flavorful fermentation.

3. create the chocolate dough: once the bulk fermentation is complete, gently divide the dough in half. in a separate small bowl, whisk together the unsweetened cocoa powder, 1/4 cup granulated sugar, and 1/4 cup warm water until a smooth paste forms. add this chocolate paste and the melted butter to one half of the dough. gently knead or fold the chocolate mixture into this dough half until the color is uniform. try not to overmix, as this can toughen the dough.

4. shape the marble loaf: on a lightly floured surface, gently flatten both the plain and chocolate doughs into rough rectangles of similar size. place the chocolate dough on top of the plain dough. gently roll them together a few times, then fold the dough in half, and roll again. repeat this folding and rolling process 2-3 times to create distinct marble swirls. avoid overworking it, as you want clear streaks, not a fully blended color. shape the marbled dough into a tight log or boule.

5. second proof (final rise): place the shaped dough into a lightly floured proofing basket (banneton) or a bowl lined with a floured kitchen towel, seam-side up. cover with plastic wrap or a damp cloth and let it proof at room temperature for 1-2 hours, or until visibly puffy and springs back slowly when gently poked. if you refrigerated your bulk fermented dough, you might need a longer second proof.

6. bake the bread: preheat your oven to 450F (230C) with a dutch oven or baking stone inside for at least 30 minutes. carefully transfer the dough from the proofing basket into the hot dutch oven. score the top of the dough with a sharp knife or razor blade (a simple “x” or a few parallel lines work well). cover the dutch oven and bake for 20 minutes. remove the lid, reduce the oven temperature to 425F (220C), and continue baking for another 20-25 minutes, or until the crust is deeply golden brown and the internal temperature reaches 200-210F (93-99C).

7. cool completely: carefully remove the bread from the dutch oven and transfer it to a wire rack. 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. slicing too early can result in a gummy texture.