Bring the kombu and water to a boil over medium heat. This should take about 10-12 minutes.
When about to boil remove and reserve kombu, add katsuoboshi and bring back to a boil, about 10-20 seconds.
Turn off heat, skim foam and let stand 3 minutes.
Strain and use or refrigerate.

To make a second stock for simmered dishes or recipes that require a less refined stock, simmer the used kombu and katsuoboshi in the same volume of water for 10 minutes. Strain and use or refrigerate.