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1
Score each chestnut lightly across the flat side. Submerge in boiling water for 5–10 minutes until the outer shell (onigawa) softens. Working one chestnut at a time and keeping the rest in the hot water, remove the outer shell with a paring knife, then peel away the inner skin (shibukawa) completely. Any trace of shibukawa left on the flesh will permeate the finished rice with bitterness. Transfer peeled chestnuts immediately to a bowl of cold water; hold there until all are peeled. The flesh should be cream-coloured and clean, with no grey or brown streaks of inner skin remaining.
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2
Rinse the rice in several changes of cold water until the water runs clear. Drain thoroughly. Place rinsed rice in a donabe clay pot or heavy-lidded rice cooker. Add the dashi or water, sake, salt, and light soy sauce. If using kombu, add it now. Stir once to combine the seasoning liquid with the rice.
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3
Drain the soaked chestnuts and lay them across the surface of the seasoned rice in a single layer. Do not stir them in. The chestnuts must sit on top so they steam in place during cooking — this is where the dish lives or dies: chestnuts disturbed before or during cooking break apart, losing both shape and the gentle sweetness they release whole into the rice beneath them.
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4
If cooking in a donabe: bring to the boil over medium heat with the lid on, then reduce to the lowest possible heat and cook for 12–15 minutes. If a rice cooker is used, set to the standard white-rice cycle. In either vessel, resist lifting the lid during cooking. When done, the steam escaping from the lid will have slowed to a near stop and a faint, dry, slightly caramelised scent — okoge — may be detected at the base of the donabe pot.
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5
Remove from heat. Rest, lid on, for exactly 10 minutes. The residual steam finishes the rice and firms the chestnuts. Open the lid: the rice surface should appear dry and glistening, each grain distinct.
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6
Using a wide rice paddle (shamoji), fold the chestnuts gently into the rice with a cutting-and-turning motion — never stirring or pressing. The chestnuts must hold their shape within the finished rice. Portion into lacquer or ceramic bowls and finish with a sprinkle of black sesame seeds or goma shio. The sesame should be perceptible on the nose before the bowl reaches the mouth.