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1
Rinse the azuki beans thoroughly and cover generously with cold water. Bring to the boil, drain, and discard the first water. Cover with fresh cold water and simmer over low heat until the beans are completely soft throughout — 60 to 90 minutes. Press a bean between two fingers; there must be no chalky, raw resistance at the centre. Under-cooked beans produce a grainy, indigestible paste.
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2
For koshi-an: pass the cooked beans through a fine sieve or tamis, discarding all skins, until the paste is completely smooth. For tsubu-an: mash the beans partially, retaining the skins for visual and textural complexity — the choice is made on the formality of the occasion and the intended aesthetic. Place the chosen paste in a wide, heavy pan over low-medium heat.
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3
Add the sugar to the bean paste in the pan. Stir continuously over medium heat, folding the paste away from the base and sides. Cook until the paste pulls cleanly from the sides and base of the pan and holds its shape when a spoon is drawn through it. Do not reduce the sugar quantity — the concentration is both flavour and preservative. The paste should be dense, glossy, and deep red-brown in colour.
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4
Dissolve the kanten in the 500 ml of water in a separate saucepan, bring to the boil, and stir until completely dissolved — agar will not dissolve below boiling point. For neri yōkan use 2–3 g; for mizu yōkan use 1–1.5 g. The kanten ratio determines the density of the finished block entirely. Stir the anko paste into the dissolved kanten liquid over low heat, combining until uniform.
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5
This is the step on which the dish lives or dies. Remove the pan from the heat and allow the mixture to cool to 80 °C before pouring into the prepared mould. Kanten sets at 40 °C; pouring at exactly 80 °C is the operational margin — too cool produces texture irregularities and a stratified set; too hot accelerates skin formation on the exposed surface. Use a probe thermometer. Pour smoothly in a single continuous motion, tap the mould once to settle the surface, and do not disturb it again.
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6
Refrigerate the filled mould for a minimum of two hours until the block is completely firm and cold. Do not cut while warm — hot yōkan produces ragged edges and an inconsistent cross-section. When set, the block should be firm, glossy, and hold a clean right angle when unmoulded. For mizu yōkan, serve directly from the refrigerator, ice cold.
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7
Unmould and slice with a single clean downward stroke of a thin blade — no sawing motion. The cut face should be perfectly smooth for koshi-an; for tsubu-an, the bean skins should be visible as distinct flecks. Serve in measured rectangular portions on a lacquered or ceramic plate alongside a bowl of freshly whisked, unsweetened matcha or gyokuro.