If Making Levain: Mill wheat berries, and mix levain ingredients in a small container, leaving for 5 hours at 25°C/77°F (so a warm room).
Mix 820 grams bread flour, 66 grams whole wheat flour, cocoa powder, and 650 grams of water, mixing with hands until no dry bit are left. Cover for 2 hours (time it for the last 2 hours of levain, if using) with plastic or damp tea towel.
When levain is ready, or after dough has autolysed, add salt, chocolate chips, and levain or sourdough starter. Add remaining water (about 100 g) but slowly as it may not all be needed.
Mix all ingredients together by hand, pinching and folding, until well incorporated. Cover.
During bulk fermentation (4 hours) give the dough 6 sets of stretch and folds, the first 3 in 15 minute intervals and the last 3 in 30 minute intervals.
Lightly flour your working surface and pour out the dough. Cut dough in half with knife.
Shape each half into a rough round shape and leave for half an hour uncovered.
Shape dough into round or oval shape an put into floured proofing baskets.
Cover with large plastic bags, seal and refrigerate overnight.
In the morning take out your loaves and set oven to 450°F.
Line loaf pans with parchment paper and carefully transfer dough over into pans. Chop chocolate bar and gently press into the tops of the loaves.
Bake loaves for 45-50 minutes until loafs are a golden hue. You can also check for an internal temperature of at least 190°F.
Cool on rack for 30 minutes.