1. 1. Combine the Wet Ingredients: In a large mixing bowl, whisk together the active sourdough starter, warm milk, granulated sugar, egg, and vanilla extract until well combined and slightly frothy. The mixture should look homogenous, with no streaks of egg yolk.

2. 2. Add Dry Ingredients and Form Dough: Gradually add the all-purpose flour and salt to the wet ingredients. Mix with a wooden spoon or your hands until a shaggy dough forms. Turn the dough out onto a lightly floured surface and knead for about 5-7 minutes until it becomes smooth and elastic. It should spring back slightly when poked.

3. 3. Incorporate Softened Butter: Add the softened unsalted butter to the dough and continue kneading for another 3-5 minutes until the butter is fully incorporated and the dough is soft and pliable. Place the dough in a lightly oiled bowl, cover it with plastic wrap or a damp towel, and let it rest in a warm place for 1-2 hours, or until it has visibly relaxed and slightly increased in size. This isn’t a full proof, but a relaxing period for easier handling.

4. 4. Prepare the Chocolate Swirl Filling: In a small bowl, whisk together the unsweetened cocoa powder and powdered sugar. Set aside.

5. 5. Assemble the Swirl Bread: On a lightly floured surface, roll out the rested dough into a large rectangle, roughly 12×18 inches. Brush the entire surface of the dough with the melted butter. Evenly sprinkle the cocoa and powdered sugar mixture over the buttered dough, leaving a small border around the edges. If using, sprinkle the chocolate chips over the cocoa mixture.

6. 6. Roll and Shape: Starting from one of the longer sides, tightly roll the dough into a log. Pinch the seam closed to seal it. Carefully transfer the log to a parchment-lined baking sheet. Using a sharp knife, make cuts along the top of the log, about 1 inch deep and 1 inch apart, creating a decorative pattern. Alternatively, you can twist the log into a spiral or braid if you prefer a more intricate design.

7. 7. Bake the Bread: Preheat your oven to 375 F (190 C). Bake the chocolate sourdough swirl bread for 30-35 minutes, or until it’s golden brown on the outside and sounds hollow when tapped on the bottom. The chocolate filling might ooze slightly, which is perfectly normal and adds to the rustic look.

8. 8. Cool and Serve: Once baked, remove the bread from the oven and let it cool on a wire rack for at least 15-20 minutes before slicing and serving. This allows the internal structure to set and the flavors to fully develop.