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1
Combine the flour, salt, and warm water in a bowl. Mix until a shaggy dough forms, then knead on an unfloured surface until the dough is smooth and supple. Wrap and rest for 30 minutes at room temperature. The dough is ready when it yields to gentle pressure without springing back immediately.
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2
Divide the rested dough into four portions of approximately 60–70 g each. Roll each portion into a rectangle approximately 25 cm long, 5 cm wide, and 3 mm thick. The surface should be even and the edges reasonably straight.
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3
This is where the dish lives or dies. Hold one rectangle at both short ends. Lift it clear of the surface, then slap the centre firmly against the counter while simultaneously pulling the two ends apart with steady, even tension — the biang biang sound confirms correct impact. Repeat the slap-and-pull motion, working along the length, until the noodle stretches to approximately 60 cm. The edges will tear into a slightly ragged fringe; this is correct. The noodle must remain a flat belt — if it narrows to a rope, the dough has not rested sufficiently.
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4
Drop the stretched noodle immediately into a large pot of rapidly boiling water. Cook for 2–3 minutes. The noodle is done when it is cooked through but retains a definite chew at its thickest point.
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5
While the noodle cooks, place the chilli flakes, minced garlic, black vinegar, soy sauce, and sesame seeds together in the serving bowl. Heat the oil in a small pan until it shimmers and begins to smoke lightly — at least 180 °C. Pour the hot oil directly over the chilli-garlic mixture in one steady stream; it will sizzle violently and the chilli will bloom and darken. The bloom is complete when the sizzling subsides and the kitchen smells of toasted chilli.
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6
Drain the noodle directly into the bowl over the sauce. Toss to coat thoroughly. Top with sliced cucumber or fresh vegetables and serve immediately. The noodle should glisten and carry an even, rust-red coat of sauce.