Liúshā Bāo (流沙包) is the lava custard bun, a steamed leavened wrapper sealed around a salted-egg-yolk custard that softens to a flowing core when the bun is opened warm. It is a fully enclosed bun rather than an open sandwich, but it belongs in the steamed-bun family alongside its savory and sweet siblings, and the angle is entirely the contrast between the wrapper and the molten center. The dough is soft, pale, and faintly sweet; the filling is a rich, slightly grainy custard built on salted egg yolk, butter or lard, sugar, and milk powder, set firm when cold so it can be wrapped, but loosening to a running stream of sweet-savory liquid the moment heat hits it. The whole effect depends on serving it warm so the inside actually flows.
The build is a wrapped-and-pleated bun with a chilled filling. A yeast-leavened wheat dough is mixed, proofed, and rolled into rounds thicker at the center than the rim. The custard is cooked down with mashed salted egg yolk, sugar, fat, and milk powder until smooth and rich, then chilled hard so it can be portioned into solid balls and sealed cleanly inside the dough; the edges are gathered and pinched shut at the crown. The buns proof briefly again, then steam over high heat until they swell and set matte white while the cold custard inside just begins to soften. Good execution shows in the flow and the seal: a wrapper that is soft and fluffy with a fine crumb, a top that is closed and did not split or weep, and a center that pours, golden and slightly salty against the sweet, the instant the bun is broken. The failure modes are specific. Under-proofed dough steams up dense and gummy and dulls the contrast; an over-steamed bun goes wet and wrinkled; a custard not chilled hard enough leaks during shaping and the filling cooks into the dough instead of staying a separate molten pocket; too little salted yolk and it tastes flatly sweet with none of the savory edge that makes the style work; served gone cold, the lava sets back to a stiff paste and the whole point is lost.
It shifts mostly by the richness of the custard and the strength of the salted yolk. More yolk and fat give a deeper, glossier, more savory flow; a lighter formula reads sweeter and milder. Some kitchens pour in custard loose enough to gush, others keep it thicker so it oozes slowly, a different pacing on the same idea. The sweet bean and lotus-paste buns are set fillings that do not flow and work on different logic, so they belong in their own articles, as do the savory pork bāozi and the steamed-flat clamshell bun. What holds liúshā bāo together is the contrast itself: a soft pale steamed wrapper sealing a salted-yolk custard that runs molten when it is opened warm.