Mix starter, flour, and water and stir well until thoroughly mixed. Add it to a clean jar and cover it with a loosely balanced lid or damp towel. Place it in a warm spot until it has at least doubled.
Dough
Once the levain is ready, bloom the cocoa. In a large bowl, combine cocoa powder and boiling water. Whisk into a smooth chocolate mixture.
Pour over the cold water, then add sugar, salt, vanilla, and all-purpose flour. Add in the risen levain. Use a fork to mix it all together into a shaggy dough, then switch to using your hands until it's all combined.
Bulk fermentation
After the starter has been incorporated, cover the bowl and let the dough rest for 10 minutes, then perform a set of stretches and folds. Using wet hands, take one side of the chocolate sourdough dough and stretch it up, then pull it over itself. Turn the dough a quarter turn and repeat. Do this on all sides of the dough. This is one 'set.'
Perform five more sets over the next 2 hours, one every 20 minutes.
After this, let the dough rise in a warm spot, ideally around 77°F/25°C, until it nearly doubles in size, around 4 hours. This will take longer if your room is cold. You can create a warm proofing spot by placing the bread dough in a turned-off oven and adding a dish of boiled water at the bottom. Replace the water as it cools down.
Shaping
Line a banneton basket or bowl with a clean kitchen towel and flour it well.
Flour a work surface. Tip your dough from the bowl onto the work surface and gently form it into a rectangle. Take care not to squash the dough too much at this shaping stage. Add 1/2 the chocolate chips onto the bottom half of the dough. Fold the top half of the dough over the chocolate chips. Press the remaining chocolate chips on top of the dough.
Take the top right corner of the rectangle and fold it down towards the center of the dough. Press it gently with your fingers to seal it in place. Repeat with all the remaining corners to create a dough parcel that encases the chocolate chips. Flip the dough ball upside down so it's smooth side up.
Cup your hands around the dough, with your pinky fingers touching the work surface. Gently move your hands in a circular motion, allowing the dough to rotate between them. Apply slight downward pressure to help shape the dough into a smooth, tight ball.
Add the shaped dough into the floured basket. If the dough spreads out in the basket, you can stitch it together by grabbing a little bit from the top left and a little from the top right and bringing them together to meet in the middle. Repeat all down the length of the dough.
Cover with a floured kitchen towel and place it in the refrigerator for at least 8 hours (or up to 24 hours.)
Baking
Preheat the oven and Dutch oven to 450°F/230°C for at least 30 minutes until well-heated.
Remove the Dutch oven and flour the bottom well. Take the dough from the fridge and carefully flip it out of the basket into the Dutch oven. If baking this in a large pot, it's best to tip your dough onto parchment paper. This way, you can lower it into the pot.
Brush it with a little flour (this flour is optional). Score the dough using a razor blade or a very sharp knife.
Bake in the pot covered with the lid for 20 minutes. Remove the lid and bake uncovered for 15-20 minutes, depending on your preference.
Let the chocolate sourdough cool to room temperature on a wire rack before slicing.
Notes
Mocha sourdough
Swap the 100g boiled water for 100g hot coffee instead to bring some coffee flavour.