Add ground caraway seeds (and whole, if using) to the dry ingredients from the dough recipe. Mix the dough as instructed in the recipe directions and allow to bulk ferment until doubled in size, an hour or two. Refrigerate overnight.
The next day, remove ⅔ of the dough and place it onto a well-floured counter. To the other ⅓, add the molasses and cocoa powder and mix completely. Dump this dough onto a well-floured counter. Knead each piece of dough until smooth and place it on a parchment-lined baking sheet. Cover loosely with plastic wrap and freeze for 15-20 minutes. This will make the dough less sticky and easier to shape.
Remove the dough from the freezer and roll or pat the light dough on a well-floured surface into a ½ to ¾-inch thick rectangle wide enough to fit into an 8½ by 4-inch loaf pan. Roll or pat the dark dough into a ¼ to ½-inch thick rectangle that's a little less wide. Place it on top of the light dough and press all around to adhere, using a rolling pin if need be.
Roll up dough as tightly as possible, just like you would for cinnamon rolls. Place the loaf into a greased 8½ by 4-inch loaf pan, seam side down. Cover loosely with plastic wrap and place in a warm, draft-free area and proof until nearly doubled in size.
Preheat oven to 450° F and place a shallow pan on the bottom rack.
Using a lame or sharp knife, slash the top of the dough in 5-6 places to about ½-inch deep. Place the loaf in the oven and pour 1 cup of very hot tap water into the shallow pan. Bake for 15 minutes and then reduce temperature to 400° F. Bake for another 30-35 minutes, or until a thermometer inserted into the center registers 200° F. If the bread is getting too dark on top, cover loosely with foil halfway through baking.
Remove from oven and cool in pan 10 minutes. Turn out of pan and cool completely before slicing. Bread will keep at room temperature, well wrapped, for a few days. For longer storage, cut into slices and flash freeze on a baking sheet about one hour, wrap in plastic wrap, and place in a ziptop bag or sealed container. Frozen slices will last in the freezer for 2-3 months.