Boil noodles according to package. Drain and immediately rinse under cold running water or rest in an ice bath until completely cold.
Make the Sauce
Whisk together tahini, soy sauce, chili oil, rice vinegar, sesame oil, sugar, and garlic. Add cold water one tablespoon at a time until the sauce is smooth and pourable. It should coat a spoon but not be gluey.
Toss
Add noodles to the sauce and toss until every strand is fully coated. The sauce should cling heavily and look dark and glossy.
Plate and Finish
Finish with ribboned scallions, sesame seeds, and a heavy chili oil drizzle that pools around the base.