Folding matcha into the bean paste turns an pan from plain sweet into something with a green, slightly bitter spine. The baseline bun calls for sweetened bean paste; this version blends in stone-ground matcha, the powdered green tea whose grassy astringency cuts straight through sugar. The roll itself is the usual soft, lightly sweet enriched bun, but the filling now has tension in it, a sweetness checked by tannin so it does not read as flatly candy. The bread still carries the load and the paste still earns the bread, but the matcha gives the whole thing an adult, tea-leaning edge that the plain version does not have.
The craft is in dosing the tea and protecting its color. Matcha is bitter and bracing in quantity, so the paste, usually a milder shiro an base that lets the green read clearly, has to be balanced so the tea is present without turning harsh or chalky. Good matcha is vivid green and aromatic, and heat dulls both, so the powder is folded into the paste rather than baked hard, and a careful baker keeps the dose high enough to taste and low enough to stay pleasant. The bun wants the familiar tender crumb, thin top, and a clean seam against a paste that should be smooth and even. A good one is fragrant, balanced, gently bitter behind the sweetness. A poor one is either a faint green tint with no tea flavor at all, or a dusty, over-bitter filling that has lost the point of being sweet bread.
Among the an pan family this is one of the flavored editions, distinct in intent from the plain red bean styles, the smooth koshian and chunky tsubuan, and from the pale, gentle shiro an it is often built on. Its natural seasonal counterpart is the salted, floral sakura version, which pulls in the opposite direction. That one deserves its own article rather than being crowded in here.