Place the kombu in a pot with the water. Bring up to 90°C and pull to 1 side. Allow the kombu to float to the surface and strain through a fine mesh sieve into a clean pan. Bring back up to 90°C and add the bonito; pull the pan off the heat. Allow the bonito to sink to the bottom of the pan. Strain through a fine mesh sieve into a clean pan. Store in an airtight container until needed.