Báijí Mó (白吉馍) is not a sandwich but the bread that makes one, the round flat wheat bun that gets split and packed with braised meat to become roujiamo. This article covers the mó on its own terms, because the entire character of the finished sandwich is decided here, in a plain disc of dough with no filling at all. The defining trait is a deliberate two-texture build: a firm, lightly crisp shell around a soft, layered interior, with a faint seam ring on the side that tells you exactly where to cut.
The craft is in the dough and the bake. A low-leavened wheat dough is worked until smooth, rested, then shaped into pucks that are often coiled or folded so the inside sets in thin sheets rather than one dense mass. Traditionally the mó is started on a flat griddle to color and firm both faces, then finished against the wall or floor of a hot clay oven so it puffs and dries to a clean shell while the center stays tender. Done well it comes out pale gold with a tiger-faint surface, a crust that gives a quiet crack under the thumb, and an interior that pulls apart in soft layers and holds its shape when split. Done poorly it shows the failure modes plainly: under-baked, it stays gummy and collapses around hot meat into paste; over-baked or all-griddle, it turns into a hard biscuit that fights the filling instead of cradling it; skip the layering and the inside is a solid wall with nowhere for juice to go.
From there it shifts by region and by method. The Shaanxi style runs firmer and crisper, built to stand up to a wet braise without falling apart, while softer local versions lean closer to a plain bun. Some kitchens bake purely in the clay oven, others rely on the griddle and a covered finish, and the wall thickness of the shell is tuned to how saucy the intended filling is. The point of the mó is structural restraint: it is mild on purpose so the braised pork, beef, or cumin lamb that goes inside reads at full strength. Where the bread is split and filled, that assembly is its own preparation, the roujiamo, and the donkey-meat and other regional fillings each deserve their own article rather than being crowded in here.