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1
Do not wash the potatoes. Heat water to 70°C and submerge the unwashed sweet potatoes for 30 minutes, maintaining the temperature as closely as possible. This activates amylase enzyme activity at its peak range of 60–70°C, converting maximum starch to malt sugars before the oven's higher heat can denature the enzymes. The potatoes are ready to proceed when they have been held at 70°C for the full 30 minutes — no earlier.
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2
Preheat the oven to 150°C. Remove the potatoes from the soak and pat dry. Wrap each potato individually and tightly in aluminium foil. Place directly on the oven rack or a baking tray and roast for 100 minutes. The foil retains moisture and allows even heat penetration without surface caramelisation at this stage. The potato is ready to unwrap when it yields to firm pressure at its thickest point — it should feel uniformly soft with no resistant core.
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3
This is where the dish lives or dies. Unwrap the foil entirely and increase the oven temperature to 200°C. Return the bare potatoes to the oven for 15 minutes. The skin must caramelise to a papery, taut, glazed surface — this is the characteristic of properly finished yaki-imo. If the foil is left on, the skin steams rather than caramelises and the defining textural and flavour contrast of the dish is lost. The potatoes are done when the skin is visibly tightened, darkened at the ridges, and the caramel aroma is clearly present.
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4
Remove from the oven. Split each potato open lengthwise with a single press — do not cut fully through. Allow 3–5 minutes of rest with the potato split open so that interior steam escapes and the texture transitions from moist to creamy-firm. For vendor-style service, transfer immediately to a paper bag after splitting; the bag absorbs excess moisture and the trapped steam continues the interior cooking. Serve when the interior surface has set from silky-wet to silky-firm.