Step 1Scrape pork belly skin with knife to remove impurities. Rinse and drain.Step 2Bring 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.Step 3Combine salt, five-spice powder, and pepper in a small bowl.Step 4Place pork belly skin side up on a tray. Pierce the skin all over with a fork.Step 5Flip 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.Step 6Pat dry rub evenly over the meat and sides of pork belly.Step 7Place 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.Step 8When ready to roast, brush skin with vinegar. Then sprinkle kosher (coarse) salt over the top.Step 9Pour 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.Step 10Move 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.Step 11Allow roasted pork belly to rest for 10 minutes. Remove charred bits by scrapping with a knife. Cut into bite size pieces.Step 12Serve immediately with mustard and sweet chili sauce if desired.