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1
Wet the interior of the moulds with cold water and set aside. This prevents the set jelly from bonding to the surface and allows clean unmoulding.
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2
Combine the kanten and water in a saucepan. Bring to the boil over moderate heat, stirring steadily, and hold at a gentle boil until the kanten is fully dissolved — the liquid should be completely clear, with no granules visible when a spoonful is held to the light.
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3
Add the sugar and salt. Stir until both are dissolved, then reduce the heat to low. Add the koshi-an in portions, stirring after each addition until the paste is fully incorporated and the mixture is uniform and smooth — no streaks of pale liquid or lumps of paste should remain. This is where the dish lives or dies: too much agar at this stage produces a firm, dense result indistinguishable from standard yokan; the ratio must be held to 50–70% of conventional yokan agar, and the bean paste must be entirely smooth and skin-free, or the defining silkiness is forfeit.
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4
Cook the combined mixture briefly over low heat, stirring continuously, until it is homogeneous and just beginning to steam. Do not boil vigorously once the bean paste is added. Remove from the heat.
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5
Pour the mixture into the prepared wet moulds. Allow to cool to room temperature, then transfer to the refrigerator. Refrigerate for a minimum of 2 hours. The jelly is set when it holds its shape on unmoulding but trembles visibly when the plate is moved.
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6
Unmould onto cold plates or serve directly in glass vessels. The jelly should be cold to the touch, smooth, and semi-translucent at its edges. Serve immediately from the refrigerator.