Mántou (馒头) is the plain steamed bun, a leavened wheat dough shaped and cooked in steam with no filling at all, soft and pale and deliberately neutral. On its own it is bread, not a sandwich, but it earns a place here because it is the workhorse base of a whole range of split-and-stuff hand-held eating: a mántou torn open or sliced and packed with whatever savory thing is to hand. The angle is the wrapper as a clean canvas. The entire usefulness of a mántou turns on a tender, faintly chewy, only-just-sweet crumb that adds structure and softness without contributing flavor of its own, so it can carry strong or fatty fillings without fighting them.
The build, as a sandwich base, is a split-and-fill. The dough is a yeast-leavened wheat dough, kneaded smooth and proofed until light, shaped into a dome, roll, or block, and steamed over high heat until it swells and sets matte white with a fine even crumb. To eat it stuffed it is sliced part-way or all the way through, sometimes lightly re-toasted or pan-warmed for a firmer face, and packed with a filling that supplies the salt, fat, and moisture the bun lacks. Good execution starts with the bun itself: soft and springy with a clean wheat taste, not gummy, collapsed, or wrinkled, and sturdy enough when split to hold a load without tearing. The failure modes are specific and show before any filling goes in. Under-proofed dough steams up dense and tight; over-steamed dough goes wet and shrunken with a sticky skin; a bun steamed unevenly has a doughy raw streak that turns to paste against a warm filling; a stale mántou eats dry and crumbly and makes whatever is stuffed in it seem like the only thing worth tasting.
It shifts mostly by shape, by how it is finished, and by what it is split to hold. Domed buns, rolled cylinders, and square blocks behave a little differently when sliced, the block giving the flattest face for a sandwich. Some kitchens pan-fry or griddle the steamed bun to crisp a golden surface, a different texture on the same neutral base, and a sliced-and-fried mántou is a common vehicle for sweet or savory loads. The filled and pleated steamed buns, the bāozi, are a different preparation that seals the filling inside rather than being split around it, and the soft steamed clamshell bun, the gua bao, is its own form; both belong in their own articles rather than being grouped in here. What holds mántou together as a category is exactly its plainness: a soft, pale, neutral steamed wrapper that exists to be the bread for everything stronger packed into it.