Wipe kombu - First wipe the kombu with a damp cloth to remove white powder on the surface, which is mostly salt that can make your dashi too salty.

Simmer kombu - Place wiped kombu in a pot with 4 cups water. Let soak 30 minutes then slowly bring to a low simmer. Just before boiling, remove kombu with tongs and discard. This infuses the water.

Steep bonito - Add your bonito flakes and turn off the heat. Let bonito steep 10 minutes then strain through a fine mesh strainer or cheesecloth. Discard solids.

Freeze dashi - Pour strained dashi into ice cube trays and freeze until solid.

Blend cubes - Pop frozen dashi cubes into a blender or food processor. Blend, pulsing, until a fine powder forms. This may take a few minutes.

Store - Transfer finished dashi powder into an airtight container. Store in a cool, dry place for up to 6 months.