In a mixing bowl, sift the all-purpose flour, salt, and baking soda together until well combined.
In a separate bowl, mix the water with kansui (or the baking soda if using) until dissolved.
Gradually add the water mixture to the flour blend, stirring with a fork until a shaggy dough begins to form.
Turn the dough out onto a clean surface and knead for about 10-15 minutes until it becomes smooth and elastic. If the dough is too sticky, sprinkle with a little additional flour.
Shape the kneaded dough into a ball, wrap it in plastic wrap, and let it rest for at least 30 minutes at room temperature.
After resting, divide the dough into 2-4 portions to make it easier to roll out. Roll each portion out on a floured surface to a thickness of about 1/8 inch.
Dust the rolled dough lightly with cornstarch to prevent sticking, then fold it into thirds and slice with a sharp knife into thin noodles (around 1/4 inch wide).
Unravel the noodles and dust with more cornstarch to keep them separated.
Bring a pot of water to a boil and lightly cook the noodles for about 2-3 minutes or until they are tender but still chewy. Drain and rinse with cold water to stop the cooking process.
Notes
Serve with your favorite toppings like green onions, soft-boiled eggs, and nori.