Dissolve the sourdough starter in the water to help distribute it evenly.

Add all the remaining ingredients except the butter and mix until no dry flour remains. Cover and let the dough rest while you prepare the dark dough.

After a short rest, knead the dough and incorporate the butter. Continue kneading until the dough becomes smooth and only slightly tacky.

Place the dough in a lightly oiled container, cover, and let it ferment until it roughly doubles in size. This may take about 8–9 hours at 69°F (20°C) or 4–6 hours in a warmer environment (do not exceed 79°F / 26°C). See notes.

Dissolve the cocoa powder in about 50 g of the water until smooth and free of lumps.

In a separate bowl, dissolve the sourdough starter in the remaining water.

Add the cocoa mixture and the rest of the ingredients (except the butter) and mix until no dry flour remains. Cover and let the dough rest while you knead the light dough.

Once the light dough is finished and set to ferment, knead the dark dough and incorporate the butter. Continue kneading until smooth and slightly tacky.

Place the dough in a lightly oiled container, cover, and let it ferment under the same conditions as the light dough. This may take about 8–9 hours at 69°F (20°C) or 4–6 hours in a warmer environment (do not exceed 79°F / 26°C). See notes

Once both doughs have finished bulk fermentation, lightly flour your work surface. Using a rolling pin, roll each dough into a rectangle about ½ inch thick and roughly as wide as your baking pan. Thicker dough will create a more defined swirl pattern.

Try to keep both rectangles similar in size (approximately 9 × 17 inches / 22 × 40 cm). Lightly prick the surface of each dough with a fork to prevent large air pockets from forming between the layers.

Lightly brush the surface of the light dough with water (or use a spray bottle), then place the dark dough on top. Use just enough water to help the layers adhere.

Gently press the doughs together to ensure full contact. Starting from the shorter side, roll the dough into a log and pinch the seam to seal.

Place the dough seam-side down in a lightly oiled 9 × 5 inch (23 × 13 cm) loaf pan. Cover with plastic wrap or a shower cap.

Let the dough proof until it becomes puffy and rises to about the height of the pan or slightly above. At 73°F (23°C), this may take about 4 hours. See notes.

When the dough is approaching the rim of the pan, begin preheating the oven to 350°F (175°C).

Once the dough has risen about 1 cm above the rim of the pan, gently brush the surface with milk or egg wash.

Bake for 35–45 minutes, or until the internal temperature reaches at least 204°F (96°C) and the crust is well browned.

Remove the bread from the oven and let it cool in the pan for about 10 minutes, then transfer it to a rack to cool completely.