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1
Combine the joshinko, caster sugar, and nikki in a bowl. Add the water gradually, mixing until a smooth, cohesive dough forms. The dough should be soft and pliable but not sticky to the touch.
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2
Press the dough into an even sheet approximately 5 mm thick on a piece of dampened muslin or parchment. Place over a steamer basket and steam over boiling water until the sheet is fully cooked through and has taken on a translucent, mochi-like quality — this is where the dish lives or dies: under-steaming leaves a raw, grainy texture; correct steaming produces the characteristic soft, yielding sheet that will fold without cracking.
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3
Remove the dough sheet from the steamer and allow it to cool just enough to handle. While still warm and supple, dust the work surface lightly with additional joshinko to prevent adhesion. Cut or portion the sheet into squares of approximately 8–9 cm per side.
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4
Place a small, firm mound of koshian — approximately 15 g — at the centre of each square. The anko must be dry enough to hold its shape; excess moisture will transfer to the dough and accelerate softening.
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5
Fold each square diagonally into a triangle, bringing two opposite corners together over the filling. Press the edges firmly; the mochi stickiness of the dough seals the parcel without adhesive. The edges should hold cleanly and the filling should be fully enclosed with no splitting at the seams.
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6
Arrange the finished triangles on a tray lined with parchment and hold at cool room temperature. Taste for the balance of warm cassia spice against the sweet anko: the cinnamon note should be assertive and warm, not faint. Consume within 2–3 days; the dough toughens noticeably by day three.