Nǎi Huáng Bāo (奶黄包) is the custard bun, a soft steamed wheat bun sealed around a sweet egg-and-milk custard that turns molten or thick-set in the steam. It is fully enclosed rather than open like a sandwich, but it earns a place here because it sits squarely in the steamed-bun family alongside the savory baozi and the split-bun forms, and because its whole character is the same dough-as-envelope logic carried into dessert. The angle is the wrapper as a soft case for a flowing center: everything turns on a pillowy, faintly sweet bun that stays tender and intact while holding a rich custard that should be smooth, not grainy, and warm enough to soften when the bun is broken open.
The build is a wrapped-and-pleated bun, not a folded one. A yeast-leavened wheat dough, often enriched with a little milk and sugar, is mixed, proofed until light, then divided and rolled into rounds thicker at the center and thinner at the rim so the gathered top does not set into a hard knot. A portion of custard, a cooked paste of egg yolk, sugar, butter or milk, and often a starch, chilled firm so it can be handled, sits in the middle; the edges are pulled up and folded into a spiral of pleats closed cleanly at the crown, or smoothed into a plain round. The buns proof again briefly, then steam over high heat until they swell, set, and turn matte white while the custard softens inside. Good execution shows a wrapper that is soft and fluffy with a fine even crumb, a sealed top that did not split or sink, and a custard that is smooth, glossy, and sweet without being cloying, oozing or yielding when the bun is torn. The failure modes are specific: under-proofed dough steams up dense and gummy; an over-filled or badly pinched bun bursts and the custard leaks into the steamer; a custard cooked too hard sets to a dry plug with no flow; too much sugar and it turns sickly rather than rich.
It shifts mostly by how the custard is set and what is added to it. A firmer, sliceable custard makes a tidy bun; a softer, salted-yolk-enriched version gives the molten, sandy-sweet style that runs when broken. Salted duck egg yolk worked into the custard is a common richer reading, deepening it toward savory-sweet. The same steamed-dough logic carries the savory pork and vegetable baozi, the red-bean and lotus-paste sweet buns, and the pan-fried bun forms, each its own preparation that deserves its own article rather than being crowded in here. What keeps nǎi huáng bāo its own entry is the pairing at its core: a soft, pale, pleated steamed wrapper sealing a smooth sweet egg custard meant to soften and flow when the bun is opened.