Combine the flour, rye flour, black cocoa powder, water, active sourdough starter, and optional maple syrup or brown sugar. Mix until no dry flour remains.

Cover and let the dough rest for 30 minutes.

Sprinkle the salt over the dough and mix until fully incorporated.

Over the next 2 hours, perform 3 to 4 sets of stretch and folds or coil folds every 30 minutes.

During your final fold, or while laminating the dough if you prefer, gently fold the chocolate chips into the dough until evenly distributed.

Continue fermenting until the dough has increased in volume by about 80 to 90%, depending on your dough temperature.

Pre-shape the dough into a round and let it rest for 20 minutes.

Complete your final shaping and place the dough seam-side up into a floured banneton.

Cover and refrigerate for 12 to 16 hours.

Bake in a preheated Dutch oven or covered baking vessel at 475°F (246°C).

Bake covered for 30 minutes

Remove the lid and bake for 20 minutes, or until the crust is deeply browned.

Allow the loaf to cool completely before slicing.