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1
Score each chestnut across the flat side with a sharp knife, cutting through the outer shell (oni-gawa) without cutting the flesh. Place in a saucepan, cover with cold water, add a generous pinch of salt, and bring to a simmer. Cook for 5 minutes. Working with the chestnuts still hot — do not allow them to cool — peel away both the hard outer shell and the papery inner membrane (shibukawa). Any chestnut that resists peeling should be returned briefly to the hot water. The cook is done when both shells come away cleanly from every piece.
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2
Bring the dashi to a gentle simmer in a wide saucepan. Add the peeled chestnuts and simmer briefly — no more than 2 to 3 minutes — until the outer starch layer is just set. Remove the chestnuts with a slotted spoon and reserve. Do not discard the dashi. This step decides the dish: chestnuts that are simmered too long in liquid before the rice stage will collapse during cooking and produce a starchy, lumpy result; those simmered too briefly will become waterlogged and flavourless. The chestnut surface should be just opaque and firm to the touch.
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3
Combine the drained rice and the reserved chestnut dashi in a heavy pot or rice cooker, adding further plain dashi to bring the total liquid to the standard proportion for the rice quantity. Season with 1 tsp salt. Nestle the briefly simmered chestnuts across the surface of the rice without submerging them. Cover tightly and cook by the absorption method. The rice is done when all liquid is absorbed, the grains are distinct, and a faint crackling sound ceases — indicating no remaining free moisture in the pot.
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4
Remove from heat and allow the pot to rest, covered, for 10 minutes. Uncover and fold the rice gently once with a shamoji (rice paddle), taking care not to break the chestnuts. Each piece should remain whole, slightly firm at the centre, and mealy-sweet. The rice grains should be individual and lightly perfumed with chestnut starch sweetness.