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1
Combine the warabiko, sugar, and water in a heavy-based saucepan. Whisk until the starch is fully dissolved and no lumps remain. The liquid will be milky-white and opaque. Set over medium heat.
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2
This is where the dish lives or dies. Stir continuously and without pause with a wooden spoon or heatproof spatula, reaching every part of the base and corners of the pan. As the mixture heats, it will begin to thicken unevenly and then rapidly. Continue for 10–15 minutes. The mixture is ready when it has transitioned entirely from milky-white to a deep, translucent grey-brown and has pulled into a single stiff, cohesive, gel-like mass that holds its shape briefly before slumping.
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3
Immediately pour and scrape the cooked mass into a wet, plastic-wrap-lined tray. Smooth the surface with a wet spatula. The mass will be extremely stiff and resistant — press it flat to an even thickness of approximately 2–3 cm.
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4
Refrigerate for 30–60 minutes until fully set and cold throughout. The surface should feel firm to the fingertip but yield immediately under light pressure — not rubbery, not liquid.
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5
Lift the set block from the tray using the plastic wrap. Cut into 2–3 cm cubes with a wet knife. The cut faces should be dark grey-brown and faintly glossy.
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6
Arrange the cubes on a chilled plate or bowl. Dust generously with sifted kinako, then drizzle kuromitsu directly over the top at table. The warabi mochi is ready when a cube dissolves on the palate with no starchy residue — immediate yielding, then nothing.