Make the Sweet Levain:In a clean jar, mix the sourdough starter, bread flour, granulated sugar, and water for the sweet levain.Cover and set in a warm location (between 75-80ºF) for 4-5 hours until doubled and bubbly.Alternatively, skip making a sweet levain and use 90g of active sourdough starter below and add 10 more grams of sugar to the loaf.

Bloom the Cocoa Powder and Autolyse:Mix the cocoa powder and sugar together in a small bowl. Pour freshly brewed hot coffee into the cocoa powder to bloom it and stir into a hot mocha mixture.About an hour before the levain is ready, mix together the bread flour, whole wheat flour, hot chocolate-coffee mixture, and warm water in a mixing bowl.Use a dough whisk or your hands to mix together the flour and water just until it comes together, cover, and rest for an hour.

Add Levain, Salt, and Mix:Add all of the levain into the bowl and dimple it into the dough. Then, stretch and fold the dough for a few minutes to incorporate.Cover and rest in a warm location for 30 minutes.Sprinkle the salt on top. Stretch and fold the dough for about 5 minutes until thoroughly mixed and the salt dissolves. While mixing, if the dough seems stiff and you think the dough can handle it, you can add another 10g of water.Cover and rest in a warm location for 30 minutes.

Bulk Fermentation, Add Chocolate Chunks, & Folds:After 30 minutes, perform one set of stretch & folds, folding in the chocolate chunks in between each fold.Separated by 30-45 minutes or so, perform a couple more sets of stretch and folds on the dough during bulk fermentation. Rest for the remainder of time (about six hours total after mixing in the salt).At the end of bulk fermentation, the dough should be smooth, feel full of air, have visible bubbles, and wobble if shaken. The dough should almost double. This dough can be sluggish, so extend fermentation if needed.

Shape:Lightly flour the top of the dough and the counter. Gently loosen the dough from the sides of the mixing bowl and turn the dough out onto the counter.With the help of a bench scraper, shape the dough into a batard (oval) or boule (round). If there are any chocolate chunks on the surface, try to stick them back into the dough so they don't burn during baking.Lift it up and place it into a floured banneton.

Cold Overnight Proof:Cover and place the banneton into a refrigerator to proof overnight and for up to two days.

Bake:Place the empty Dutch oven with lid in the oven and preheat for an hour at 500°F (260ºC).Once preheated, remove the banneton from the refrigerator and turn the dough out onto a piece of parchment paper. Score the dough with a sharp bread lame about ¼-½" deep.Transfer the scored dough to the Dutch oven and cover it with the lid.Bake at 500°F (260ºC) for 20 minutes. Remove the lid, lower the oven temperature to 450ºF (232ºC) and bake for about 13-15 minutes with the lid off.Remove from the oven and cool on a wire rack for at least an hour before slicing.