Zhīmajiàng (芝麻酱) is sesame paste, and as a sandwich entry it stands in for the whole class of northern hand-held foods built around that paste as the binding, dressing, and dominant flavor. It is ground roasted sesame worked into a thick, oily, deeply nutty spread, looser and darker than tahini, and it earns a place here because it is the element that turns a plain split bread or rolled flat dough into a filled, sauced thing eaten in the hand. The angle is the paste as the load-bearing flavor: get a sandwich that leans on zhīmajiàng right and the sesame carries every bite with a roasted, bittersweet richness; get it wrong and it is either a dry seized clump or a greasy slick with no grip.
The build is a spread-and-fold or spread-and-roll assembly where the paste does most of the work. Raw zhīmajiàng out of the jar is stiff and separated, so it is first loosened, usually let down with a little water, sesame oil, or stock and beaten until it turns from a tight mass into a pourable, glossy sauce, then seasoned, commonly with salt and sometimes sugar, soy, or vinegar depending on whether the result leans savory or sweet. That loosened paste is spread thick onto a split shāobǐng or a warm flat bread, or smeared down a sheet of dough before it is rolled, with the bread itself griddled or baked so its surface can hold the sauce without going soggy. Done well it shows paste loosened to a clinging coat that spreads evenly and stays put, a roasted sesame depth that reads clearly without turning acrid, and a bread crisp enough on the outside to contrast the soft sauced interior. Done poorly the failures are plain: paste added straight and unloosened so it sits in a dry, claggy lump that tears the bread, paste cut with too much oil or water so it runs out the sides and leaves the bread slick and tasteless, sesame ground from old or scorched seed so the whole thing eats bitter, or so much sauce the bread collapses to paste underneath it.
It shifts by what the loosened paste is paired with and on which bread it is carried. A plain savory smear inside a sesame-topped shāobǐng is the simplest form, a sauce-and-bread sandwich with nothing else; layering in sugar pushes it toward a sweet snack, while adding pickled vegetable, scallion, or a fried dough stick turns it into a fuller filled bread with the sesame as the unifying note. The flatbreads and rolled doughs it dresses, the shāobǐng and the rolled wheat flats, are their own preparations with their own structural rules and get their own treatment rather than being crowded in here. What fixes this entry is the paste itself as the defining element, the roasted-sesame binding that makes a split or rolled bread into a sauced sandwich.