Place the konbu in a small saucepan with the water. Bring to a boil. Remove the konbu and reserve for another use.
Remove from the heat, stir in the bonito flakes, and allow them to settle to the bottom of the broth.
Strain through a damp-cloth-lined sieve; gather up the cloth and squeeze out the excess liquid. Discard the bonito flakes. Stir in the soy sauce, mirin, and salt.</