Step 1

Soak the kombu: Combine the water and kombu in a 1-quart or larger saucepan and soak the kombu for at least 8 hours or overnight.

Step 2

Bring it to a boil: Place the saucepan over medium-high heat and bring to a simmer. Continue to simmer and then remove the kombu from the water just before it comes to a full boil.

Step 3

Add the mushrooms and simmer: Add the shiitakes, if using, and let the broth come to a rapid simmer. Continue simmering for about 1 minute.

Step 4

Steep the shiitake mushrooms: Remove the saucepan from heat and let the shiitakes steep in the broth for 5 minutes more.

Step 5

Strain the broth: Remove the shiitakes from the broth (save for making miso soup or another recipe). Pour the broth through a fine-mesh strainer set over a large bowl.

Step 6

Use or store the broth: The broth can be used immediately or refrigerated or frozen.