Feed starter. The night before, feed your sourdough starter so you have 100 grams of active and bubbly starter the next day. Combine 30 grams of starter with 40 grams of unbleached all-purpose flour and 30 grams of filtered water.
Autolyze. In a large bowl, add the active sourdough starter and water. Stir to combine. Next, add the all-purpose flour, stir to combine, and let the mixture sit for 10 minutes.
Mix dry ingredients. In a small bowl, add the whole wheat flour, salt, sugar, vanilla bean powder, and cocoa powder. Stir until fully combined.
Make dough. Add the cocoa powder mixture to the flour/sourdough mixture, using a spatula to mix everything together. Once it gets too hard to use the spatula, turn the dough out onto a clean, dry surface and knead it to make sure everything is mixed in.
Rest. Cover the bowl and let it sit covered for 30 minutes.
Stretch and fold. Wet your hands with warm water, pick up one side of the dough, stretch, and fold it over. Repeat this process 3 or 4 times. Flip the dough over. Cover the dough and let it sit for 30 minutes. Repeat the stretch and folds 2 more times. (This dough is dryer, so it may not stretch out very far.)
First rise. After the last stretch and fold session, cover the dough and let it sit at room temperature for 6-8 hours or until it has doubled in size.
Add chocolate chips. Turn the dough out onto a clean, dry surface. Fold the chocolate chips into the dough by adding about a third at a time, gently folding the dough each time. (Try to avoid pressing too hard when folding so you don’t remove all of the air inside the dough.)
Second rise. Shape the dough into a ball, then place it in a well-floured banneton and cover with plastic. Place it in the fridge for 10-12 hours (overnight is best).
Preheat oven. Preheat the Dutch oven to 425℉ for a minimum of 30 minutes. (This helps make sure that the Dutch oven is at temperature.)
Prep loaf. Take the dough out of the fridge and turn the dough out onto a piece of parchment paper or a silicone bread mat. Using a bread razor, score the dough down the center, shift the dough 90 degrees and score the dough again making a + sign or an X.
Bake. Place the dough into the hot Dutch oven. Cover and bake at 425℉ for 20 minutes. Remove the lid and reduce the heat of the oven to 375℉. Bake for another 20 minutes.
Cool and slice. Remove the bread from the oven and place it on a wire cooling rack. Let it cool completely (at least 1 hour) before cutting into it.
Notes
To speed this recipe up, you can add 1 teaspoon of instant yeast and reduce the rise time to 6 hours total.