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1
Light a charcoal brazier and allow the coals to reach a steady, moderate heat with no raw flame. Distribute the coals evenly. The correct stage is a grey-ashed ember bed that glows orange when fanned — not a roaring fire.
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2
Set the sweet potatoes directly on the coals or on a grate immediately above them. Rotate a quarter-turn every three to four minutes. This step decides the dish: patient, even rotation over moderate heat is what separates a sweet potato that is soft and uniformly caramelised throughout from one that is charred outside and raw within — rush the heat or neglect the turning and the dish is lost. Continue until a skewer or thin blade meets no resistance at the centre.
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3
Lay the corn, husks intact, beside or above the coals. Rotate on the same rhythm as the sweet potatoes. The husks trap steam against the kernels during the first phase of cooking. When the outer husks are fully blackened and papery, peel them back to expose the kernels directly to the charcoal for the final stage.
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4
Continue rotating the husked corn over direct charcoal until the kernels are spotted with deep brown-gold char in places and the corn smells of toasted sugar. The Maillard reaction on the natural sugars gives the characteristic caramel-smoke aroma that signals readiness.
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5
For the northern style, remove the corn and sweet potatoes from the heat. Finish with fine salt only; the salt amplifies the inherent sweetness of both vegetables. For the Sichuan variant, blend the softened butter with doubanjiang and brush across the hot corn kernels, then scatter toasted sesame seeds.