Place chili flakes, crushed Sichuan peppercorns, sesame seeds, and minced garlic in a heatproof bowl.
In a small saucepan, heat neutral oil with cinnamon stick and star anise over medium heat until fragrant and shimmering, about 3–4 minutes.
Carefully pour hot oil over the chili mixture. It should sizzle immediately.
Let cool, then remove whole spices once infused.
Cook the Noodles
Bring a large pot of salted water to a boil.
Cook noodles until just tender. Fresh noodles take 2–3 minutes; dried 4–6 minutes.
Reserve about 1/4 cup noodle cooking water before draining.
Drain thoroughly.
Combine
In a large mixing bowl, combine noodles, chili oil, soy sauce, black vinegar, sugar, and sliced scallions. The sauce should taste bold and slightly sharp.
Toss vigorously to emulsify.
Notes
Dark soy is not a substitute for black vinegar.
High heat when pouring oil is key for aroma.
Sauce should taste slightly intense before noodles absorb it.