Lightly oil two mixing bowls and set aside.

In a large bowl or stand mixer, combine:

Sourdough starter, warm water, honey, rye flour, bread flour, and salt.

Knead with a dough hook on medium-low for 7 minutes (or by hand for the same time).

Divide the dough in half. Transfer one half to a prepared bowl and cover.

Add cocoa powder and molasses to the other half. Mix until fully combined.

Transfer the dark dough to the second bowl, cover both, and set aside.

Let both doughs rest for 30-60 minutes.

Perform stretch and folds every 30 minutes for 4-6 hours, or until the dough doubles in size.

Line a loaf pan with parchment paper.

Roll both doughs into 6x10-inch rectangles on a lightly floured surface.

Place the dark dough on top of the light dough, then roll tightly into a cylinder.

Pinch the seam closed and roll again for extra tension.

Place the seam side down in the loaf pan.

Cover with plastic wrap and refrigerate overnight.

Preheat the oven to 425°F.

Remove the dough from the fridge and score the top with diagonal slashes.

Cover with a second loaf pan (or tent with foil) and bake for 30 minutes.

Remove the cover and bake for another 25-30 minutes, until golden brown.

The bread is ready when it reaches 205-210°F internally.

Remove the loaf from the oven and let it cool for 30-60 minutes before slicing.

Serve fresh, toasted, or as the base for a delicious Reuben sandwich.