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1
Place the dried sweet potato noodles in a large bowl and cover generously with cold water. Soak for 30 minutes. They are ready when pliable and uniformly softened throughout, with no hard or opaque core when bent.
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2
Bring a large pot of water to a rolling boil. Into each serving bowl, measure the chilli oil with its sediment, Chinkiang vinegar, soy sauce, sesame paste, Sichuan peppercorn oil, and sugar. Stir briefly to combine. This is where the dish lives or dies: the balance of sour, spicy, numbing, and savoury must be present in the bowl before the noodles arrive — adjust vinegar or chilli oil now, not after, as dilution from the noodles will flatten any correction made later.
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3
Drain the soaked noodles and lower them into the boiling water. Cook for 3–5 minutes, stirring once to prevent clumping. Lift a strand and bite through: the noodle should be cooked through and yielding yet retain a distinct, elastic chew, with the slight translucency characteristic of sweet potato starch.
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4
Ladle 1–2 tablespoons of the hot noodle cooking water into each prepared bowl and stir to thin the sauce to a loose, cohesive consistency. Drain the noodles and divide among the bowls, laying them directly into the sauce.
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5
Top each bowl with fried peanuts, ya cai, fried soybeans, sliced scallion, and fresh coriander. Finish with an additional drizzle of chilli oil. The surface should show distinct pools of red oil against the dark broth, with visible texture from the toppings.