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1
Lightly wet a rectangular mould or silicone loaf tin with cold water. Set aside. This ensures clean release when the yokan is cold.
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2
Combine the kanten powder and the water in a saucepan. Bring to a gentle simmer over low heat, stirring steadily, until the powder is entirely dissolved. Do not allow a rolling boil: over-boiling drives off setting power. The liquid is ready when it is fully clear and no granules are visible against the light.
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3
Add the caster sugar and stir until dissolved, keeping the heat low. The liquid should remain limpid and slightly viscous; it must not colour.
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4
This is where the dish lives or dies. Add the koshian in several additions, whisking each addition until completely smooth before proceeding. Any lump, skin fragment, or grain of unincorporated bean paste will set permanently into the finished block and cannot be corrected. If doubt exists about smoothness, pass the mixture through a fine-mesh sieve before the next step. The combined mixture should be a uniform, glossy, deep-brown fluid with no visible texture.
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5
Pour the mixture into the prepared mould in a single motion. Tap the mould once on the surface to settle any air pockets. Allow to cool to room temperature, then refrigerate until fully set. The block is ready when it holds a clean, straight edge on pressing with a fingertip and the surface yields no liquid.
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6
Unmould only when completely cold. Invert the mould onto a clean board; the block should release without tearing. Slice into pieces 5–8 mm thick with a single, clean downward stroke of a wet knife. Warm yokan tears and loses its clean surface; if the block resists a clean cut, return it to the refrigerator.