Xiǎolóngbāo (小笼包) is the soup dumpling, a thin unleavened wheat skin pleated shut around seasoned pork and a set jellied stock that melts back to hot broth in the steamer. It is a dumpling by construction rather than a sandwich, fully enclosed and eaten whole, but it earns a place here because it sits at the structural edge of the steamed filled-bun family the catalog tracks and because some kitchens serve a larger soup-filled version split or stuffed into bread. The angle is the skin as a pressure vessel: the entire character of a xiǎolóngbāo turns on a wrapper rolled thin and translucent that must be strong enough to hold a load of liquid and meat through the steam without tearing, then yield cleanly to the first bite.
The build is a pleated parcel with a liquid core, not a folded one. A cold-water wheat dough is worked until smooth and rolled into small rounds left thicker at the center and very thin at the rim. The filling is seasoned minced pork combined with an aspic, a stock set firm with gelatin or skin, so it goes in solid and renders back to soup under heat. A portion sits in the middle, the edges are gathered and folded into a tight ring of fine pleats twisted closed at the crown, and the dumplings are steamed briefly over high heat in a bamboo basket until the skin sets translucent and the aspic liquefies inside. Done well it shows a wrapper that is thin and even with a clean pleated top that did not split, a generous pool of hot, clear, well-seasoned broth held intact, and pork that is tender and savory. The failure modes are specific: a skin rolled too thick reads doughy and dead at the pleated knot; rolled too thin or overstuffed it tears in the basket and the soup is lost before it reaches the table; too little aspic and there is no broth at all, just a dry meatball in a wrapper.
It shifts mostly by filling and by size. The classic pork-and-aspic build is the reference; crab roe and crab meat folded in give a richer, briny version; a few regional styles add a touch of sweetness to the broth. The oversized soup bun, large enough to drink through a straw, is a related but distinct preparation, as is the steamed leavened bāozi that shares the pleated-and-steamed logic but uses a soft risen wrapper rather than this thin unleavened skin. Those each deserve their own article rather than being crowded in here. What holds xiǎolóngbāo together as a category is exactly that engineered skin: thin, strong, and translucent, sealing meat and a melting pool of soup in a single pleated parcel.