In separate bowls, whisk together water, sourdough starter, vanilla paste, and bread flour for your vanilla dough; combine Dutch cocoa and black cocoa powders in another bowl for the chocolate dough. Let both doughs rest for 60 minutes.

Employ the stretch and fold technique for each dough every 30 minutes, repeating this process for a total of 2 hours.

On a floured surface, stack your vanilla and chocolate doughs together, gently rolling them into a log shape. Allow the dough to rest.

Form the rested dough into your desired loaf shape. Let it rest for 30 minutes.

Transfer the shaped dough to a banneton, then leave it at room temperature for 2-3 hours.

Preheat your oven to 450°F (230°C). Bake the dough on a hot baking stone, adding steam to achieve a crispy crust. Bake for 35-40 minutes.

Once baked, transfer the loaf to a cooling rack and allow it to cool completely for 2 hours before slicing.