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1
Combine the kanten powder and water in a medium saucepan. Bring to a full boil over medium heat, stirring continuously until the kanten is completely dissolved — no granules visible, the liquid perfectly clear and smooth. Incomplete dissolution here will produce a grainy final texture. The liquid should look like clean glass.
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2
Add the sugar to the dissolved kanten liquid and stir until fully incorporated. Reduce the heat to low, then add the anko in stages, stirring steadily to combine without lumps. This is where the dish lives or dies: the anko must be evenly suspended throughout the liquid before the temperature falls, and the mixture must remain fluid — if it tightens too quickly, the final texture will be uneven and the set inconsistent.
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3
Remove the saucepan from the heat. Allow the mixture to cool, monitoring with a thermometer, until it reads between 60 °C and 65 °C. At this exact window, pour the mixture into a lightly wetted rectangular mold in one steady motion. Pouring above 65 °C risks uneven setting as the agar begins to seize on contact with the cooler mold.
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4
Allow the mold to cool at room temperature until the surface is no longer steaming and the jelly has lost its sheen, then transfer to the refrigerator. Chill until fully set. When unmolded, the mizu yokan should tremble slightly — a gentle, restrained quiver that confirms correct agar concentration and moisture level.
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5
Unmold onto a cutting board. To slice, draw a very thin sharp knife through the block in a single clean stroke — do not saw. The faces should be smooth and glassy, holding their edge without crumbling. Serve chilled.