Bāozi (包子) is the steamed filled bun, a leavened wheat dough gathered and pleated shut around a filling, then cooked in steam until it sets soft and white. It is usually fully enclosed rather than open like a sandwich, but it earns a place here because some styles are served split or stuffed after steaming, and because it sits at the structural root of gua bao and the whole split-bun family. The angle is the dough as a steamed envelope: the entire character of a bāozi turns on a pillowy, slightly sweet wrapper that has to stay tender and intact while sealing a hot, often juicy core.
The build is a wrapped-and-pleated bun, not a folded one. A yeast-leavened wheat dough is mixed, proofed until light, then divided and rolled into rounds that are thicker at the center and thinner at the rim so the gathered top does not become a hard knot of dough. A portion of filling, commonly seasoned pork, sometimes with a gelatin that melts to juice, or vegetables, or a sweet paste, sits in the middle; the edges are pulled up and folded into a spiral of pleats that close cleanly at the crown. The buns are proofed again briefly, then steamed over high heat until they swell, set, and turn matte white. Good execution shows a wrapper that is soft and fluffy with a fine even crumb, a sealed pleated top that did not split or sink, and a filling that is hot and well seasoned with the juices held inside rather than blown out. The failure modes are specific: under-proofed dough steams up dense and gummy, an over-stuffed or badly pinched bun bursts at the seam and leaks, and a wrapper rolled flat and even all over leaves a thick, doughy plug where the pleats pile up.
It shifts by filling, by size, and by how it is finished. Large single buns with a savory pork or vegetable core are an everyday form; small soup-juiced versions are a more delicate build; sweet bean or custard fillings push it toward dessert. Some shops pan-fry the steamed or raw buns to crisp a golden base, a different texture on the same envelope. The split or after-stuffed styles are where it edges closest to a sandwich, and the soft folded clamshell bun that is steamed flat and packed with braised pork, the gua bao, is its own preparation that deserves its own article rather than being crowded in here. What holds bāozi together as a category is the steamed leavened wrapper sealing a filling, soft, pale, and pleated.