Score pork belly skin without cutting into the meat.
Season meat side with soy sauce, garlic powder, black pepper, allspice, and brown sugar.
Pat skin dry, rub with vinegar, then salt (and baking powder if using).
Place skin-side up on a rack over a tray and refrigerate uncovered overnight.
Preheat oven to 300°F (150°C) and wipe excess salt off the skin.
Roast pork skin-side up for 90 minutes.
Increase oven to 425°F (220°C) and roast 25–30 mins until skin is puffed and crisp.
If needed, broil for 3–5 mins to blister skin.
Rest for 10–15 mins before slicing with a serrated knife.
(Optional) For glazed version, brush a honey-soy-ginger-hot sauce mix on the meat (not skin) in the last 10 mins.