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1
Combine the kuzu starch and cold water in a heavy saucepan. Whisk until the starch is fully dissolved and the liquid is uniformly cloudy with no dry lumps remaining. Add the sugar and stir to dissolve. The mixture must be entirely cold and smooth before any heat is applied — adding kuzu starch to warm or hot water produces irreversible clumps.
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2
Place the pan over medium heat. Begin stirring immediately with a wooden spoon or heat-resistant spatula and do not stop. This is where the dish lives or dies: if stirring pauses for even a moment, lumps form and cannot be rescued. Work the spoon continuously across the base and into the corners of the pan as the mixture heats.
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3
Continue cooking and stirring. The mixture will pass through a milky, increasingly viscous stage. Gelatinisation is complete when the mass turns fully transparent and shimmering — no white opacity remains. The gel will be thick, glossy, and pull away cleanly from the sides of the pan.
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4
Pour immediately into lightly wetted moulds or a flat tray. Smooth the surface. Allow to cool briefly at room temperature, then refrigerate for 1 to 2 hours. The kuzu mochi is ready when it is fully firm, cool to the touch, and holds its shape when unmoulded — it should be noticeably firmer and more resistant than gelatin.
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5
Unmould and cut into squares or cubes with a wet knife. Arrange on plates or in shallow bowls. Serve the kinako and kuromitsu in separate vessels alongside; the diner applies both to each piece individually. The mochi should be cool, clear, and faintly trembling — warm kuzu mochi has an unpleasant, slack texture and must not be served before it is properly chilled.