Strained smooth, koshian gives an pan its silkiest form. Where the baseline bun calls for sweet red bean paste in general, this version specifies koshian, the preparation in which the cooked adzuki beans are pushed through a sieve and the skins removed so the paste comes out fine, uniform, and dense. The roll around it is the familiar soft, lightly sweet enriched bun, but the filling has no grain at all. It melts rather than chews. The bread still carries the sweetness and the paste still justifies the bread, but the experience here is about smoothness, a clean continuous texture from crumb to core with nothing to interrupt it.
The work is in the straining and the moisture. Proper koshian is laborious, the beans simmered soft, sieved, then rinsed and pressed to drop the skins before being cooked back down with sugar to a pliable, glossy paste that holds its shape without weeping into the crumb. It should taste cleanly of sweetened adzuki, more refined and less earthy than the chunky style, with no graininess on the tongue. The bun wants a tender, slightly chewy interior and a thin burnished top, and a tight seam matters because a loose, wet paste will turn the underside gummy fast. A good one is a quiet study in uniformity. A poor one is either dry and pasty, where the koshian has been overcooked into a brick, or thin and weepy, where it soaks the bread and the bun goes soggy from the inside.
This is the elegant, tea-house-leaning member of the an pan family, the smooth foil to the rustic tsubuan with its visible beans. Beyond those two the line continues into shiro an made from white beans for something paler and gentler, the grassy matcha version, and the salted, floral sakura edition. Each of those changes the flavor enough that it deserves its own article rather than being crowded in here.