Make the dough: In a large bowl, whisk flour and salt. Make a well and slowly pour in warm water, mixing with chopsticks or a fork until a shaggy dough forms.
Knead: Turn dough onto a work surface and knead until smooth and elastic, about 10 minutes. Cover with a damp towel and rest at room temperature for 30 minutes.
Portion and oil-rest: Cut dough into 8 equal pieces. Roll each into a 6-inch log, then lightly coat each piece with oil. Arrange on a plate, cover, and rest for 2 hours (or refrigerate up to 24 hours; bring to room temperature before pulling).
Prep sauce and toppings: In each serving bowl, combine soy sauce, black vinegar, rice vinegar, sesame oil, sugar, minced garlic, chili flakes, and crushed Sichuan peppercorns. Set aside. Chop green onions and rinse the bok choy.
Cook greens: Bring a large pot of water to a boil. Blanch bok choy for 1–2 minutes until bright green and tender-crisp; remove and drain.
Pull the noodles: Working with one dough log at a time, flatten slightly with your hands, then stretch from the center outward. Slap the dough against the counter a few times to help elongate. Pull into a long, wide noodle about 1–2 inches wide. (Optional: tear down the center lengthwise for the traditional look.)
Boil: Drop the noodle into boiling water immediately. Cook 2–3 minutes until it floats and turns slightly translucent. Remove with tongs and transfer to the sauce bowl. Repeat with remaining pieces, cooking one at a time for best texture.
Sizzle oil: Heat peanut oil until shimmering and very hot (but not smoking). Carefully pour the hot oil over the chili flakes/garlic mixture in each bowl to bloom the aromatics.
Toss and serve: Add bok choy, green onions, and sesame seeds. Toss well so the sauce coats the noodles. Top with cilantro if using and serve immediately.