There isn't a whole lot of cocoa powder that goes onto these short ribs, but this is one case where a little goes a long way. The chocolate flavor infuses the cooking liquid and the meat soaks it all up overnight, resulting in a complexity of flavor that just wouldn't be there without the combination of chocolate and port.
Preheat the oven to 300 degrees.
Heat the oil in a sauté pan or Dutch oven over high heat. Season the short ribs with salt and pepper, to taste. When the oil is hot, add the short ribs and sear on all sides, about 10-12 minutes. Coat the short ribs with the cocoa powder, turning them with tongs.
Add the chicken stock and port and bring to a simmer. Cover with a lid and place in the oven until the short ribs are tender, about 3 hours. Remove from the oven, let cool in the liquid, and place in the refrigerator overnight.
The next day, remove the solidified fat from the top of the cooking liquid. Remove the short ribs from the cooking liquid and set aside. Place the pot back over the stove and add the carrots, mushrooms, and potatoes. Cover with the lid, and bring to a simmer over high heat.
Then, reduce the heat to low to maintain the simmer. Cook until the potatoes are fork-tender, about 12-15 minutes, depending on the size of the potatoes. Season with salt and pepper, to taste. Add the short ribs back into the pot to warm through, about 5-7 minutes.