The key to crispy skin is removing as much moisture as possible. Pat the entire pork belly slab, especially the skin, completely dry with paper towels. For truly exceptional results, place the uncovered pork belly on a rack in the fridge for a few hours, or even overnight. This will dehydrate the skin significantly.
Flip the pork belly so it is meat-side up. Using a sharp knife, make several ½-inch deep cuts into the meat, being careful not to slice through to the skin. If using the optional meat rub, combine the garlic powder, five-spice powder, white pepper, and sugar in a small bowl. Rub this mixture all over the meat side, pushing it into the scored lines. Do not get the spices on the skin, as they can burn at high heat.
Flip the pork belly skin-side up. If the butcher hasn't already, score the skin in a diamond pattern, cutting just through the skin and into the fat layer but not into the meat. An even better method for a bubbly crackle is to use a sharp skewer or the tip of a knife to poke many small holes all over the skin. Brush the skin with the white vinegar and then rub the coarse sea salt generously all over it, working it into the scores or holes.
Preheat your air fryer to 250°F (120°C). Create a "boat" or tray for the pork belly using aluminum foil, folding up the sides to encase the meat portions but leaving the skin completely exposed. This keeps the meat juicy while the skin dries out. Place the foil-wrapped pork belly into the air fryer basket and cook for 30 minutes. This low-temperature stage renders the fat and dries the skin further.
After 30 minutes, carefully remove the pork from the air fryer. The salt on top should have formed a hard crust. Scrape off and discard this salt crust, brushing away any excess crystals. Now, increase the air fryer temperature to 400°F (200°C). Place the pork belly (without the foil) back into the basket, skin-side up. Cook for 30-40 minutes. Check on it at the 20-minute mark and then every 5 minutes after that, as air fryers can vary. The pork is done when the skin is deeply golden, bubbly, and crackled.
Remove the pork belly from the air fryer and let it rest on a cutting board for at least 10 minutes. This is a vital step that allows the juices to redistribute, ensuring the meat is tender and not dry. To slice, place it skin-side down and use a heavy, sharp knife to press firmly through the crispy skin. Cut into cubes or slices and serve immediately.