This recipe requires a thermometer for accuracy, but you can estimate temperatures using the guides in the recipe.
Yields about 2 quarts
2 (4-inch) square pieces kombu
2½ quarts water
2 cups (1 ounce) bonito flakes or katsuobushi
Put the kombu in a 4-quart saucepan, cover with the water, and soak for 30 minutes.
Set the saucepan over medium heat until the water starts to bubble gently; do not boil. This will take about 9 to 10 minutes.
Remove the kombu from the pan and discard. Increase the heat to high and bring to a boil, 5 to 6 minutes.
Reduce the heat to low and add the bonito flakes. Simmer gently, stirring, for about 10 minutes.
Strain the liquid through a fine-mesh strainer lined with cheesecloth. Store in an airtight container in the refrigerator. Use within 1 week or freeze for up to a month.