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1
Combine the kanten and water in a medium saucepan. Set over medium heat and bring to a full boil, stirring continuously until the agar is completely dissolved — the liquid must be clear, with no visible granules remaining.
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2
Add the sugar to the dissolved kanten liquid. Stir over medium heat until every grain is dissolved and the syrup returns to a boil.
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3
Add the anko paste to the pan and stir with vigour to integrate it fully into the kanten syrup. Continue cooking over medium heat, stirring without pause, until the mixture thickens to a dense, homogeneous mass that pulls cleanly from the sides of the pan. This is where the dish lives or dies: under-cooked anko at this stage produces a grainy, poorly integrated texture that no amount of chilling will correct.
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4
Remove from heat. Pour the mixture into a rinsed rectangular mould in one steady, low pass, keeping the pan close to the mould surface to suppress air bubbles. Do not rush or jolt the mould — bubbles formed at this stage are permanent. The surface should settle to a smooth, glossy plane.
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5
Allow to cool at room temperature until the yokan is fully set and firm to a light press at the centre — the surface should be dry to the touch, not tacky, and the jelly should hold its shape when the mould is tilted.
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6
Unmould and slice with a moistened blade into pieces 5–7 mm thick. The cut face should be clean and even, with no dragging or crumbling at the edge.