The tsubuan version of an pan is the one where you can see the beans. Where the baseline bun simply asks for sweet red bean paste, this reading specifies tsubuan, the chunky preparation in which whole or partly broken adzuki beans stay visible and intact in the filling. The enriched bun is the same soft, faintly sweet roll, but the paste inside has body and grain to it, a coarse jammy texture that pushes back against the tooth instead of dissolving. The bun and the paste still need each other for the usual reason, but here the paste is also doing textural work, and the contrast between pillowy crumb and resistant bean is the entire point.
The craft question is whether the beans hold. Good tsubuan is cooked so the skins stay soft but the beans keep their shape, sweetened enough to taste like a treat without burying the earthy, almost chestnut flavor of adzuki underneath sugar. It should mound rather than spread, sitting as a defined core inside the roll. The bun wants the same tender, slightly chewy crumb and thin burnished top as any an pan, with a clean seam so the coarse paste does not blow out the side during baking. Done well, you get distinct beans suspended in a glossy bind against soft bread. Done badly, the beans are mushy and indistinct, the sugar is flat and one-note, and the whole thing collapses into the same texture as a careless smooth version, which defeats the reason anyone chose tsubuan in the first place.
Within the an pan family this is the rustic, country-leaning choice, the one that tastes most like the bean itself. Its obvious foil is the strained koshian version, all silk and no grain. From there the family runs to shiro an made from white beans, the grassy matcha edition, and the salted, floral sakura one. Each of those is a real divergence in flavor and intent and deserves its own article rather than being crowded in here.