1To prepare dashi stock, gently wipe the Kombu with a damp cloth. Place the Kombu in 1.8 liters of water and let it soak for at least 30 minutes, or overnight in the refrigerator. Heat the pot over medium heat, removing the Kombu just before the water boils. Bring the water to a boil, then turn off the heat and add the Katsuobushi. Let it steep for 5 minutes, then strain the dashi through a fine-mesh sieve, discarding the Katsuobushi.
2In a separate pot, combine 1.5 liters of the prepared dashi stock with Koikuchi soy sauce, Mirin, and Sake. Bring to a gentle simmer to create the soup base.
