Mix together 50g ripe sourdough starter, 100g rye flour and 100g water. Lightly cover and leave at room temp for 12 hours to ferment.

After 12 hours, check levain for readiness: it should have risen, be very bubbly throughout, and have a mild sour aroma. If these signs are not present, leave to ferment for 30 more minutes and check again.

To a large mixing bowl, add entire levain, remaining rye flour, bread flour, gluten, onion powder, caraway seeds, salt, sugar, olive oil, milk powder, and remaining water.

Mix all ingredients together until mixed thoroughly. The dough will be somewhat sticky, but it should not be runny. Scrape the sides of the bowl to incorporate all ingredients.

Remove half of the dough to another bowl. To one of the bowls, add the 1/2 Tbsp rye flour and 1/2 Tbsp black cocoa powder. Mix until the dough is a uniform color.

Cover the bowls and set in a warm place on the counter for bulk fermentation.

Set a timer for 30 minutes. When it goes off, wet or lightly oil your hands and perform a set of stretch and folds (on both doughs). Repeat with two more sets of stretch and folds at 30 minute intervals, and then leave the doughs to sit for the remainder of bulk fermentation, 2 hours and 30 minutes more.

After four hours have passed, check the doughs for readiness: they should be puffy and visibly risen and have a lot of small bubbles, especially on the bottom (it's okay if the top isn't bubby, as long as it's puffy). If your kitchen is cold, fermentation may take longer. Leave the dough for 30 more minutes and check again.

Grease a 8.5x4.5 or 9x5 loaf pan and set aside.

Lightly flour a clean work surface or a sheet of parchment paper.

Scrape out the light dough onto your work surface and spread the dough out into a rectangle, about 10 in x 7 in or so.

Add the dark dough directly on top of the light dough and spread it out, leaving a small border.

Starting with one long side, roll the dough up. A bowl scraper can be helpful in rolling if your dough is sticking. Once the dough is all rolled into one log, tuck the ends under and transfer to the loaf pan.

Cover the loaf pan with plastic wrap and place in a warm place on the counter.

Leave on the counter to proof for 3 hours.

At the end of proofing time, the dough should be risen substantially in the pan. Toward the end of the proofing time, preheat the oven to 425 degrees Farenheit.

If desired, beat egg with 1 Tbsp water and brush onto the loaf. If desired, very lightly score the top of your loaf with a bread lame (it may very well crack if you don't).

Bake for 15 minutes at 425, then decrease oven temp to 350 and bake for 15 more minutes. Remove loaf from the pan directly to the oven rack and bake 5-7 minutes more. When finished, your loaf will be browned, have a crisp crust, and sound hollow when tapped.

Remove to a cooling rack and allow to cool completely before slicing.