Pour the dashi, soy sauce, mirin and sugar into a small saucepan. Stir well and heat gently until the sugar dissolves. Remove the saucepan from the heat and let the sauce cool completely.

Bring a large pot of water to a boil. Add the soba noodles and cook according to the package directions. Stir gently during cooking to prevent the noodles from sticking.

Taste one noodle before draining. It should be tender but still slightly firm. Avoid overcooking, because soft noodles can become mushy after chilling.

Drain the cooked noodles in a colander. Rinse them under cold running water while gently rubbing the noodles with your fingers. This removes excess starch and gives them a clean texture.

Prepare a large bowl with cold water and ice cubes. Place the rinsed noodles into the icy water for about one minute. This makes them cold, firm and refreshing.

Lift the noodles from the ice water and drain them very well. Excess water can weaken the dipping sauce. Gently shake the colander to remove as much water as possible.

Slice the green onions and cut the nori into thin strips. Grate the ginger and daikon. Keep each topping separate so everyone can add their favorite flavors.

Divide the cold soba between four serving plates. Sprinkle sesame seeds and nori over the noodles. Serve the chilled dipping sauce separately with ginger, daikon and green onion.