Bloom the yeast: Whisk the water, molasses and yeast together in a large mixing bowl. Cover the bowl and let the yeast bloom until foamy, about 15 minutes.
300 grams water, 7 grams active-dry yeast, 28 grams molasses
Make the dough: Using a stiff spatula or a wooden spoon, stir the sourdough discard, flour, spices and salt into the yeast mixture until no dry bits of flour remain. Switch to your hands and knead until a cohesive dough forms.
113 grams sourdough discard, 150 grams rye flour, 240 grams all-purpose flour, 120 grams bread flour, 1 teaspoon caraway seeds, ½ teaspoon anise seeds, ¼ teaspoon fennel seeds, ½ teaspoon coriander seeds, 9 grams salt
Knead the oil into the dough until completely absorbed. The dough should start greasy and gradually become smoother and less sticky.
28 grams vegetable oil
Make the dark rye dough: Divide the dough in half. Return half of the dough to the mixing bowl, add the cocoa powder and water into the bowl and knead until fully absorbed.
10 grams cocoa powder, 28 grams water
Strengthen the dough: Turn one portion of dough out onto a lightly floured work surface. Knead the dough by folding it in half, then pressing and pushing with the heel of your hand. Continue this process until the dough becomes smooth and elastic, about 5 to 10 minutes. Repeat with the other half.
Bulk fermentation: Round each portion of dough into a tight ball, and transfer it to a clean, lightly oiled bowl. Spread oil on top of the dough to prevent it from drying out. Repeat with the other portion.
Cover the bowls with plastic wrap and set it in a warm place until doubled in volume, about 45 minutes to 1 hour.
Prep: Preheat the oven to 400 F. Line a 9 by 4-inch Pullman pan (see note) with parchment paper or brush liberally with oil.
Final shape: Turn the dough out on a lightly floured work surface. Working with one portion at a time, divide the dough in half. Using a rolling pin, flatten the dough into a long rectangle about 4 inches wide. Repeat with the other portion of dough.
Layer one portion of chocolate dough over one portion of light dough. Starting with the shorter end, roll the dough into a tight cylinder. Place dough into the Pullman Pan. Repeat with the remaining portions of dough.
Final Proof: Cover the loaf pan with plastic wrap. Let the dough rise untouched in a warm place. The dough should fill your pan and rise about ½ an inch above the rim, about 30 to 45 minutes.
Bake: Brush the dough with vegetable oil. Score four diagonal lines on top of the dough. Bake for 20 minutes at 400 F. Without taking the loaf out of the oven, turn the temperature down to 350 F and continue baking for another 20 to 25 minutes or until the loaf registers at 200 F when probed with an instant-read thermometer.
Move the bread to a wire rack and let it cool completely, about 2 hours, before slicing.