Remove any dust or particles on kombu with a damp cloth (but leave the white powdery substances which contribute to the umami flavor in the stock). These days kombu is much cleaner and you don't really have to wipe it. Never wash under running water!
In a donabe (Japanese clay pot), put dashi kombu and 1 ½-2 cups (360-480 ml) cups water. Let the kombu soak for 1-2 hours. Or you can steep the water with kombu slowly over the lowest heat, if you didn't have time to soak.
To Prepare the Sauce
Meanwhile, make the sauce. Put sauce ingredients in a small sauce pan.
When boiling, add bonito flakes (katsuobushi).
Mix well with chopsticks and cook for 1-2 minutes.
Pour the sauce into a small serving dish. You don’t have to strain; it’s okay if some bonito flakes go into the sauce.
When the kombu dashi is ready (after 1-2 hours), bring the dashi to a boil on medium heat. If you were slowly heating up the kombu dashi (you didn't have time to soak for 1-2 hours), bring it to a boil, too. Once boiling, add sake and salt.
When boiling again, cut the tofu to any size you like (whole tofu, halves, 6 pieces, etc) and put it in the kombu dashi.
When tofu is heated through, after several minutes, add mitsuba. If you plan to use other vegetables, make sure are they cooked through.
Serve with the sauce and toppings of your choice while the tofu is hot.