To make the sauce, combine the dashi stock, mirin and soy sauce and cook until just boiling. Set aside.
Bring 8 cups of water to a boil in a large saucepan and add the noodles, stirring gently to keep them separate. Cook at a vigorous boil until the noodles are cooked but still firm, about 5 minutes. Drain in a colander and rinse under cold water.