Tiánmiànjiàng (甜面酱) is not a sandwich but the sweet wheat-flour sauce that anchors a whole class of them, the thick dark fermented paste brushed inside Peking duck wraps, stuffed into many filled flatbreads, and stirred through countless street fillings. This article covers the sauce on its own terms, because in the sandwiches it touches it is rarely a garnish and almost always the load-bearing flavor: the thing that turns plain bread, scallion, and a sliced protein into a coherent bite. The defining trait is a balance of sweet and savory built by fermentation, deep and malty rather than sugary, with a smooth body that clings to bread and slick fillings without running off.
The craft is in the ferment and the cook. A wheat-flour base is fermented with a culture and salt over a long hold until it darkens and develops a roasted, soy-adjacent depth, then it is often cooked down with sugar and oil to thicken and round it before it reaches a wrap. Used well it does specific structural work in a sandwich: a thin brushed layer on a thin pancake carries the fat of roast duck and binds the scallion and cucumber baton against the slick skin so the roll holds together and every bite is seasoned. In a stuffed bun or roll it gets mixed into the filling so the sweetness threads through the meat rather than sitting in a stripe. Done well the sauce reads balanced and glossy, salty and sweet in step, thick enough to stay where it is placed. Done poorly the failures are obvious in the finished sandwich: too heavy a hand and the paste's sweetness flattens everything else into one note, too thin and it slides to one end leaving the rest unseasoned, raw and uncooked it can taste sharp and floury against delicate fillings.
It shifts by region, ferment length, and how much sugar and oil are cooked in. Northern styles tend drier and saltier, others sweeter and looser; some cooks thin it with stock and aromatics into a dip-grade glaze for duck, others keep it stiff for stuffing. It is frequently confused with or blended alongside fermented soybean pastes, but the wheat-flour base gives it a rounder, less funky profile, which is why it suits the clean, fatty duck roll so well. The wraps and buns it defines are each their own preparations and get their own entries; what fixes the sauce here is its role as the sweet-savory backbone that holds those flatbread sandwiches together and decides how they taste.