Preheat the oven to 200°C, 400°F, Gas 6. Season the pork belly all over with some salt and freshly ground black pepper and with the five-spice powder. Put the seasoned pork belly on a roasting tray and put it in the oven. Cook it for 20 minutes to get the crackling going, then turn down the heat to 120°C, 250°F, Gas 1. Continue to cook the pork for about three hours until the meat is meltingly tender.
Remove it from the oven and allow it to cool slightly, then use two forks to pull apart the meat and the crackling, ready to go into the sandwiches.