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1
Prepare the sauce bowl before the noodles are cooked. Into each individual serving bowl measure the chilli oil with its sediment, soy sauce, Sichuan peppercorn oil, diluted sesame paste, vinegar, sugar, minced garlic, and minced ginger. Stir once to loosely combine. The bowl should be ready and waiting — the sauce is built cold, the heat of the noodles and cooking water will bring it together.
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2
Bring a generous pot of unsalted water to a full rolling boil. Cook the thin alkaline noodles until just tender but retaining a distinct springy, slightly chewy resistance — the alkaline treatment is what gives them this character, and it must not be cooked away. Lift the noodles immediately; do not allow them to sit in the water.
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3
Reserve 2 tbsp of the hot noodle cooking water per bowl and add it directly to the prepared sauce bowl. This is where the dish lives or dies: the starchy, alkaline cooking water emulsifies the sesame paste and chilli oil into a coherent, intensely aromatic sauce that is complex but fluid — looser than dan dan mian, never thick or cloying. Stir the bowl. The aroma rising from it should be immediately sharp, numbing, and sesame-rich.
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4
Drain the noodles thoroughly and transfer them into the sauce bowl. Toss so that every strand is coated. The noodles should glisten and carry the sauce without pooling at the base of the bowl.
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5
Top each bowl with ya cai, roasted peanuts, and sliced scallion. Add blanched water spinach or bok choy if using. Serve immediately — the dish is eaten at the stall, without delay.