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1
Heat the oven to 130 °C. Wrap each satsumaimo individually in aluminium foil. Place on the oven rack and roast for 90 minutes. The dish is built on this stage: slow heat converts the starches to maltose and concentrates the natural sugars; roasting at too high a temperature or for too short a time produces an ordinary baked potato, not yakiimo. At 90 minutes, pierce the thickest part through the foil with a skewer — it must meet no resistance, and a caramel fragrance should escape when the foil is opened.
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2
Remove the sweet potatoes from the foil immediately. Peel while still hot, discarding the skin. Transfer the flesh at once to a wide, warm bowl and mash vigorously with a sturdy fork or potato masher. Mashing must begin while the flesh is at its hottest; cooling causes the starch to set and resist smooth incorporation. The mash is ready to strain when no large lumps remain.
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3
Press the hot mash through a fine-meshed sieve in batches, using a bench scraper or the back of a ladle. Work in one direction to push the paste through cleanly. Discard any fibrous strings remaining in the sieve. The sieved paste should fall from the sieve in slow, heavy ribbons — silky and completely smooth to sight.
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4
Transfer the sieved paste to a saucepan over very low heat. Add the cold butter pieces and sugar while the paste is still hot. Stir continuously with a wooden spoon, folding from the base of the pan, until the butter is fully absorbed and the sugar has dissolved into the paste. If the paste is too stiff to stir, add water or dashi one teaspoon at a time — no more than 2 tablespoons total. Taste before adding sugar: natural sweetness varies by variety and harvest. The finished kinton should be smooth, dense, and pipeable — it holds a peak pressed between two fingers without slumping.
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5
To finish as kurikinton: steep two cracked dried kuchinashi pods in 3 tablespoons of hot water for 5 minutes until the liquid turns deep amber-gold. Strain and fold the coloured liquid into the kinton paste in place of plain water, one teaspoon at a time, until the traditional bright yellow-gold colour is achieved uniformly throughout. The colour should read as clean gold to the eye — not orange, not brown.
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6
Transfer the finished kinton to a piping bag fitted with a plain or star nozzle for wagashi presentation, or press into small moulds. Serve warm or at room temperature. The paste firms slightly on cooling — this is correct. A properly made kinton holds its piped or moulded shape cleanly without weeping liquid.