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Cook the noodles according to the package directions You want them cooked softer than al dente. Drain and toss with the neutral oil in a medium bowl or on a small-rimmed baking sheet. Cover and refrigerate until cold. Do not rinse the noodles to chill them faster; it removes all the starch that helps the sauce cling to the pasta.
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In a small bowl, whisk the sesame paste and the peanut butter until smooth.
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Add the sesame oil, sugar, black vinegar, dark soy sauce, Sichuan peppercorn oil, and salt and whisk again until smooth. You can also do all this in a food processor.
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Whisk in the mayonnaise and then whisk in enough water until a thin creamy, silky sauce forms. The mayo helps make an emulsion and add creaminess.
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Divide the noodles among bowls; drizzle the sesame sauce on top. Garnish with the scallions, a drizzle of chili oil, and sesame seeds.