Scrape pork belly skin with knife to remove impurities. Rinse and drain.
Bring a pot of water to boil. Blanch pork belly in boiling water skin side down for 5 minutes. Flip pork belly and allow it to blanch for another 5 minutes. Remove and pat dry with paper towels.
Combine salt, five-spice powder, and pepper in a small bowl.
Place pork belly skin side up on a tray. Pierce the skin all over with a fork.
Flip the pork belly over. Using a sharp knife cut several lines about ¼ inch deep and 1 inch apart along the length of the pork belly.
Pat dry rub evenly over the meat and sides of pork belly.
Place a wire rack on a baking tray. Transfer the pork belly skin side up onto the wire rack. Pat dry skin with paper towels. Allow it to dry uncovered overnight in the refrigerator.
When ready to roast, brush skin with vinegar. Then sprinkle kosher (coarse) salt over the top.
Pour just enough water to cover the base of the tray without wetting the pork belly. Roast in a 375°F (190°C) oven for 50 minutes. Remove from oven.
Move oven rack with mittens to the top most level close to the broiler. Return meat to the rack. With the oven door slightly ajar, turn oven to broiler (grill) setting. The pork belly skin will start to blister. Continue to broil (grill) for approximately 5 to 10 minutes or until skin is evenly blistered. Remove when done.
Allow roasted pork belly to rest for 10 minutes. Remove charred bits by scrapping with a knife. Cut into bite size pieces.
Serve immediately with mustard and sweet chili sauce if desired.