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1
Combine the warabi ko, cold water, and sugar in a heavy-bottomed saucepan. Whisk until the starch is fully dissolved and no lumps remain. The mixture will be white and opaque.
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2
Place the saucepan over medium heat and begin stirring immediately with a wooden spoon or heatproof spatula, sweeping the base and sides of the pan without pause. This is where the dish lives or dies: the high-starch mixture burns in seconds the moment stirring ceases — do not stop for any reason. Continue cooking and stirring constantly until the mixture transitions from white opacity to full, characteristic translucency — a deep, trembling, semi-clear gel. This transition is the only reliable doneness signal.
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3
Pour the translucent gel immediately into a clean container that has been set in a larger tray filled with ice water. Smooth the surface with a wet spatula. Leave to chill in the ice water for a minimum of one hour. The set jelly should hold a cube shape when cut yet tremble visibly when the dish is moved — the yururi quality.
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4
Cut the set jelly into cubes directly in the container, or turn it out onto a wet board and cube it there. Transfer the cubes to a bowl of fresh ice water to keep them cold and separate until plating.
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5
Lift the jelly cubes from the ice water and arrange in individual glass dishes. Dust generously with kinako — the dry, toasted powder should coat the surface without dissolving into the jelly. Drizzle kuromitsu over each portion. The dark molasses fragrance of the syrup meeting the nutty warmth of the kinako signals the sweet is ready.