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1
Combine flour and yeast. Add warm water gradually and bring together into a smooth dough. Knead until supple. Cover and allow to ferment at room temperature only until slightly risen and just lightened — not a full doubling. The dough must retain density; it is done when it feels marginally softer than when it started but has not become puffy or airy.
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2
Mix ground pork with soy sauce, Shaoxing wine, sesame oil, sugar, and ginger. Stir vigorously in one direction until the mixture coheres and becomes sticky. Fold in the diced aspic. Keep the filling cold — refrigerate if the kitchen is warm — so the aspic does not begin to melt before cooking. The filling is ready when it holds its shape and the aspic cubes remain distinct.
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3
Divide the dough into portions of roughly 25 g each. Roll each into a round disc, thicker at the centre than at the edge. Place a generous spoonful of cold filling at the centre. Pleat and seal the bun firmly at the top, then invert so the smooth base faces down and the pleated seam is upward. The seal must be tight — any gap will allow the melted aspic soup to escape during cooking.
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4
Place a heavy iron skillet over cold heat. Add the neutral oil, then arrange the buns seam-side up in the cold oil, close together but not touching. Begin heating gradually from cold. This is where the dish lives or dies: starting in a cold pan gives the base time to develop an even, deeply golden crust without scorching before the interior is anywhere near cooked. Do not rush the heat. The base is ready to proceed when it is firmly golden-brown and releases cleanly from the pan — heard as a dry sizzle rather than a wet sputter.
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5
Add exactly 100 ml of water to the hot pan — no more — and cover immediately with a tight lid. The water will flash to steam and cook the tops of the buns while the base remains above the waterline and continues to crisp. Steam until the water is fully absorbed and the tops are cooked through, no longer doughy to the eye, and the dough has a soft, pillowy sheen.
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6
Remove the lid. Scatter the black sesame seeds and sliced spring onion directly over the buns. Allow 30 seconds for the residual heat to just set the garnish and for any last moisture to evaporate from the base. The buns are done when the bottom crust is audibly crisp if tapped lightly with a spoon.