In a large saucepan over medium, heat the kombu and 6 cups water to just below a simmer. Remove the kombu (discard it or reserve it for another use) and bring the liquid to a boil over medium-high.

Turn off the heat, add the bonito flakes and let steep for about 2 minutes.

Pour the broth through a fine-mesh strainer set over a medium bowl. Discard the bonito and return the broth to the pan. Stir in the soy sauce, mirin and sugar; set aside.

In a large pot, bring 3 quarts water to a boil. Using your hands, add the noodles to the pot, shaking them over the baking sheet to remove excess starch. Cook, stirring occasionally, until a noodle rinsed under cold water is tender, 15 to 17 minutes. Meanwhile, bring the broth to a simmer over medium, then remove from the heat and cover to keep warm.

When the noodles are done, drain in a colander, rinse under running warm water and drain again. Divide the noodles among individual bowls. Ladle in the hot broth and sprinkle with the scallions. If desired, serve with shichimi togarashi.