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1
Rinse an 18 × 9 cm rectangular mould with cold water. Do not dry it. Set it level and within reach of the hob. The water film it carries is not incidental — it is the release mechanism.
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2
Combine the water and kanten powder in a small saucepan. Bring to a full boil over medium heat, stirring steadily. Hold at the boil for two minutes, stirring throughout. The liquid must be clear and lightly viscous — any cloudiness or undissolved granules means the agar is not yet fully hydrated and the cast yōkan will show pale, unset areas at the centre.
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3
Add the sugar and stir until completely dissolved. Add the koshi-an and stir to combine fully, working out any streaks of unmixed paste. This is where the dish lives or dies: the mixture must return to a gentle boil and be held there, stirring constantly, until it is uniform in colour — deep burgundy-brown with no lighter swirls — and flows from the spoon in a single, unbroken ribbon. Insufficient heat at this stage means the agar and paste remain imperfectly integrated, producing a yōkan with an uneven set.
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4
Remove from the heat. Pour immediately into the cold-rinsed mould in a single steady motion. The surface should settle flat without assistance. Skim any surface bubbles with a spoon.
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5
Cool at room temperature for one hour undisturbed. Rapid or uneven cooling produces surface condensation that weakens the exterior. The yōkan is ready to refrigerate when it no longer releases steam and holds its shape visibly at the edges.
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6
Refrigerate until fully set. Press the surface lightly with one finger: it should offer slight, even resistance before yielding — not collapsing immediately, not rigid. Turn out onto a board. Cut with a thin, sharp knife in a single drawing motion. The cut face should be smooth and slightly glossy; a crumbling face indicates excess agar, a smearing face indicates too little.