In a small mixing bowl, add the cocoa, sugar, orange zest, black pepper, and clove. Combine well to form a rub. Coat the pork with oil and sprinkle with the salt. Smear the cocoa rub all over the pork shoulder. Add the onion quarters and pork to a large dish and pour orange juice and red wine vinegar over the pork. Cover with plastic wrap and refrigerate for 4 to 12 hours to marinate.
Preheat the oven to 425 degrees F and arrange a rack in the middle. Let pork sit a room temperature while oven heats up, at least 45 minutes to 1 hour.
Remove pork from marinade and dry well. Put the pork in a clean, dry roasting pan, and roast until the skin is browned, about 30 minutes. Reduce the oven to 325 degrees F, scatter onions from the marinade around the pork and roast until the internal temperature registers 155 degrees F on an instant-read thermometer, about 2 to 2 1/2 to 2 hours.
Transfer the pork to a cutting board and cover loosely with foil. Allow the roast to rest for 15 to 30 minutes before carving.
Slice between the meat and cracklings and lift the crackling off in 1 piece. Cut the crackling into pieces, so they're easier to eat. Carve meat downward and against the grain and arrange on a platter. Spoon pan juices and onions over the meat and serve.