The night before, mix 50g starter, 100g dark rye flour, and 100g warm water in a jar. Cover and rest 8 to 12 hours until doubled.

Combine the remaining bread flour, dark rye flour, cocoa, and salt in a bowl. Dissolve molasses in the remaining water, then add it and all of the levain to the dry ingredients. Mix to a shaggy dough, cover, and rest 30 minutes.

Perform 4 sets of stretch-and-folds at 30-minute intervals, then let the dough bulk ferment until puffy and increased by 40 to 50 percent.

Shape the dough into a tight log, place seam-side up in a floured banneton, and cover. Proof at room temperature 1 to 2 hours until risen by 30 percent, or cold proof 8 to 12 hours in the fridge.

Preheat a Dutch oven at 450°F (230°C) for 45 minutes. Turn the loaf onto parchment, score the top, and bake covered 25 minutes. Uncover and bake 15 to 20 minutes more, until 200°F (93°C) internal. Cool completely on a rack, then slice and serve.