Put the 500g of pork skin and/or bones in a pot, cover with cold water, bring to a boil and boil for 5 minutes, then drain and rinse everything well. Return it to a clean pot with the 1,200ml of water, 2 scallions, 15g of ginger and 1 tbsp of Shaoxing wine, and simmer gently for 2–2½ hours, until the liquid is noticeably reduced and sticky. Strain into a shallow container, leave it unsalted so the filling carries the seasoning, and chill until fully set and firm. Dice it small only once it is cold. That slow reduction and shallow chilling is what gives a genuinely firm jelly that cuts into neat cubes instead of running into the pork.

While the aspic chills, mix the 400g of flour, 5g of yeast, 20g of sugar and 4g of fine salt in a large bowl. Add the 220g of lukewarm water and 10g of oil, then knead for 8–10 minutes until the dough is smooth and soft but not sticky. Cover and leave it for 1–1¼ hours, until slightly puffy; it does not need to double. Keep the dough on the drier side, because a wet dough is difficult to pleat and seal. That moderate rise is what keeps the bun thin, genuinely tender and compact instead of bready and airy.

Put the 500g of well-chilled ground pork in a chilled bowl with the chopped scallions, ginger, 2 tbsp of light soy sauce, Shaoxing wine, dark soy sauce, sugar, sesame oil, fine salt and white pepper. Stir vigorously in one direction for several minutes until the mixture turns sticky, cohesive and tacky. If it feels loose, add the 60ml of cold water a little at a time, mixing until each addition is absorbed. Fold in the 180–220g of diced aspic last and refrigerate the filling until you are ready to shape. That one-directional mixing is what emulsifies the meat so it genuinely holds the juices released later.

Punch the risen dough down lightly, then divide it into 24 equal pieces of about 27g each, keeping the pieces covered with a cloth so the surface does not dry. Roll each piece into a round wrapper about 9cm wide, leaving the centre slightly thicker and the edge thinner. Divide the chilled filling evenly between the wrappers, about 30–33g each, and cover the stack of rolled wrappers as you work. That thicker centre is what supports the melting aspic and helps prevent soup from leaking through the base.

Place a portion of filling in the centre of each wrapper and pleat the edge closed at the top in small, even folds, pinching firmly so the top is completely sealed. Do not overstuff; each finished bun should look plump, compact and dumpling-sized, with a smooth flat bottom. Set the buns on a lightly floured tray, cover loosely and rest them for 15–20 minutes. If any filling has softened while you shape, return the tray to the refrigerator for a few minutes. That tight seal is what keeps the juices inside the bun during cooking.

Heat a heavy nonstick or well-seasoned skillet over medium heat and add a thin layer of oil, using a little of the 2 tbsp for each batch. Arrange the buns with the smooth flat bottoms down and the pleated tops up, leaving a little space between them. Fry for 2–3 minutes, without moving them, until the bottoms begin to take on colour. Work in batches rather than crowding the pan, so every base sits flat against the oil. That initial fry in hot oil is what starts the golden crust before any water goes in.

Carefully add 180–220ml of water, enough to come about 0.8–1cm up the sides of the buns, and cover immediately, because it will splatter. Steam-fry for 8–10 minutes until the water is mostly absorbed and the buns are cooked through, adding a small splash more if it evaporates early. Uncover and cook 2–4 minutes longer until the bottoms are deeply golden and crisp, scattering on the sesame seeds and sliced scallions near the end, and lower the heat if the bases darken too fast. That covered steaming followed by an uncovered finish is what gives a soft pale top over a genuinely crisp base.

Stir the 4 tbsp of black vinegar and 2 tbsp of light soy sauce together in a small bowl, adding a few drops of chilli oil only if you want heat. Lift the buns out of the pan with a thin spatula as soon as the bases are crisp and arrange them crisp side up on a warm plate, with the dipping sauce beside them. Bite carefully, because the filling and any soup inside are very hot. That immediate serving is what keeps the bottoms crackling and the filling juicy.