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1
Dissolve the salt in the cold soft water. Add the salted water to the flour in a large bowl gradually, working the liquid in with the fingertips until no dry flour remains and the dough begins to come together. Turn out and knead firmly on an unfloured board for 10–15 minutes, pressing and folding repeatedly, until the surface is smooth and the dough shows no tears or rough patches. The dough will be stiff. Wrap tightly and rest at room temperature for 2–4 hours — gluten must fully relax. The dough is ready when it yields to gentle thumb pressure without springing back sharply and its surface is entirely smooth.
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2
Divide the rested dough into two equal pieces. Working one piece at a time, dust the board lightly with flour and roll the dough out with a rolling pin, rotating a quarter-turn between passes, until it reaches a uniform thickness of 3–4 mm — no thinner, no thicker. Hold the sheet up to light: it should be even with no thick ridges or thin patches. Dust the sheet lightly, fold it loosely, and cut into flat noodles of 3–4 mm width with a sharp knife. Shake the cut noodles free and dust with flour to prevent sticking.
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3
This is where the dish lives or dies. Bring a large pot of unsalted water to a vigorous, rolling boil — not a lazy simmer but a full, turbulent boil that will keep the noodles moving throughout. Lower the noodles in and cook for 10–12 minutes, stirring in the first minute to separate them. The water must not lose its boil; adjust heat accordingly. At 10 minutes, lift a noodle and bite through: the cross-section must show no white, chalky core. The cooked noodle should be firm but with a gentle, smooth chew and a slightly translucent appearance at the surface. Drain immediately.
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4
Transfer the drained noodles to a bowl of iced water and rinse thoroughly, turning them with the hands for 30–60 seconds. This firms the texture and removes surface starch. The noodles are ready when they feel cool, taut, and slick — not soft or tacky — to the touch.
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5
Drain the chilled noodles well and arrange on a flat zaru (bamboo draining tray) or a chilled plate, separating the strands so they do not clump. Serve cold, immediately, alongside cold tsuyu — both the standard soy-dashi and the sesame tsuyu may be offered. The noodle surface should appear smooth and faintly translucent; a matte, opaque surface indicates overcooking or insufficient rinsing.