Rinse the pork belly and pat dry. Place it skin-side down on a tray, and rub the Shaoxing wine into the meat (not the skin). Mix together the salt, sugar, five spice powder and white pepper. Thoroughly rub this spice mixture into the meat as well. Flip the meat over so it’s skin-side up.
So, to do the next step, you'll need a sharp metal skewer. Systematically poke holes ALL over the skin, which will help the skin crisp up, rather than stay smooth and leathery. The more holes there are, the better, really. Let it dry out in the fridge uncovered, for 12-24 hours.
Preheat the oven to 375 degrees F. Place a large piece of aluminum foil (heavy duty foil works best) onto a baking tray, and fold up the sides around the pork snugly, so that you’re creating a kind of box all around it, with a 1-inch high border going around the sides.
Brush the rice wine vinegar on top of the pork skin. Pack the sea salt in one even layer over the skin, so the pork is completely covered. Place in the oven and roast for 1 hour and 30 minutes. If your pork belly still has the rib attached, roast for 1 hour and 45 minutes.
Take the pork out of the oven, turn on the broiler to low, and position the oven rack in the lowest position. Remove the top layer of sea salt from the pork belly, and unfold the foil. Put the pork back under the broiler to crisp up. This should take 5-10 minutes. The broiler should ideally be on “low” so that this process can happen gradually. If your broiler gets pretty hot, be sure to keep the pork as far away from the heat source as possible. Keep a close eye on it to prevent burning!
When the skin has puffed up and gotten crispy, remove from the oven. Let it rest for about 15 minutes and then slice.