Cover the dried shiitake mushrooms with the water and allow them to soak for 5 minutes.
Drain the mushrooms, reserving 2½ cups / 625 ml (21.1 fl oz) of the flavorful soaking water.
Set the soaked mushrooms aside.
Gently wipe both sides of the kombu with a damp cloth to clean it without removing the white umami-rich powder.
In a saucepan, bring the reserved mushroom water to a boil over medium-high heat.
Immediately remove the saucepan from the heat.
Add the wiped kombu and the soaked shiitake mushrooms to the hot water.
Let the steep undisturbed for 5 minutes to fully infuse the broth.
Strain the liquid through a fine-mesh sieve to obtain your clear dashi.
Reserve the mushrooms and kombu for another use.
Don't boil the kombu.
Adding it to the water after it has been removed from the heat prevents the broth from becoming bitter or slimy.