Make the dashi base: in a saucepan, combine about 4 cups of cold Water with a 4–6-inch piece of soaked Kombu and let it sit 20–30 minutes; then slowly heat to just below a boil, remove the kombu, add a handful of Katsuobushi, turn off the heat and let steep 3–5 minutes before straining. If you prefer to shortcut, use ready-made Dashi instead and skip the kombu/katsuobushi step.
Turn the strained dashi into tsuyu: mix the cooled dashi with Soy Sauce and Mirin to taste (a common starting ratio is roughly 4 parts dashi : 1 part soy : 1 part mirin), then chill the sauce in the refrigerator while you cook the noodles so it’s crisp and cold when served.
Bring a large pot of water to a rolling boil and cook the Dried Soba Noodles according to package instructions (usually 4–5 minutes), stirring gently so they don’t stick. Cook until just tender but still with a little bite.
When the noodles are done, quickly drain them and rinse thoroughly under cold running water to remove surface starch and stop cooking. Then transfer the noodles into an ice bath made with cold Water and several Ice Cubes to chill completely and firm up the texture; leave for a minute or two.
Prepare garnishes while the noodles chill: thinly slice the Scallions, grate the Daikon finely and squeeze out any excess water if desired, and shred the Nori into thin strips. Place a small amount of Wasabi Paste on a separate dish or alongside the toppings.
Assemble and serve: drain the chilled noodles well and arrange them on a bamboo mat or plate. Sprinkle the shredded Nori over the top. Pour the cold tsuyu into small dipping bowls and set out bowls of sliced Scallions, grated Daikon and a dab of Wasabi Paste so each person can customize their dip.
To eat, take a portion of noodles, dip them into the chilled tsuyu (add scallion, daikon or wasabi to the tsuyu as you like), and enjoy immediately for the best texture and flavor.
