Lǎogānmā (老干妈) is the jarred chili crisp, a fried-chili-and-oil condiment rather than a sandwich, but it earns a place here because it is one of the most common things spooned into Chinese hand-held breads and buns to lift a plain or fatty filling. The angle is its role as a finishing layer. Lǎogānmā is not a sauce that cooks into a dish so much as a topping that sits on it: a slick of seasoned oil carrying crunchy fried chili flakes, fermented soybeans, and aromatics, salty and savory with a low background heat and a pronounced toasted crunch. In a sandwich it is the seasoning that turns a bland steamed bun and a bit of meat or egg into something with edge and texture.
The build, when it appears in a sandwich, is a thin deliberate layer rather than a smear. A mántou, a split mó, a folded clamshell bun, or a wheat flatbread is opened, the main filling laid in, and a measured spoonful of the crisp dragged across so the oil coats the surface and the solid bits, fried chili, soybean, peanut or preserved vegetable depending on the type, are distributed evenly. The point is contrast: the crisp supplies salt, savor, and a brittle crunch against a soft starchy bread and a soft or fatty interior. Good execution is about restraint and placement. Enough oil to gloss and season the bite, enough solids to give crunch in most mouthfuls, and the bread kept structurally intact so the oil flavors it without soaking it to a translucent patch. The failure modes are easy to read. Too heavy a spoonful and the oil bleeds through and the bread goes greasy and limp; the crunchy solids settle into one corner and the rest of the sandwich tastes only of plain oil; pile it onto an already rich, oily filling and the whole thing turns slick and one-note with nothing to push against.
It shifts mostly by which type is used and what it is paired with. The classic crispy-chili-in-oil version is the general-purpose choice; a chili-and-fermented-black-bean type reads saltier and funkier; a peanut or preserved-vegetable version brings a different crunch and a touch of sweetness. Against egg it sharpens a mild filling; against fatty braised pork it cuts the richness; against plain steamed bread it does most of the flavoring on its own. It is a condiment, not a dish, so the breads and buns it dresses, the steamed buns, the split mó, the clamshell bao, are their own preparations and belong in their own articles. What holds lǎogānmā together as a category here is its function: a salty, crunchy, oil-borne finishing layer that gives a plain Chinese sandwich its seasoning and bite.