Place 6 cups water in a large saucepan over high heat. Add kelp and bring to a boil. Immediately remove kelp with tongs (if it boils it can turn the broth bitter), reserving it for Second Dashi.Advertisement
Add the remaining 1/2 cup cold water (to cool the broth), then add bonito flakes. Return to a boil, then immediately turn off heat (if boiled, bonito can also turn the broth bitter). Let the broth rest for 2 minutes, then pour through a cheesecloth-lined sieve, making sure to avoid pressing down on the solids (which will turn dashi cloudy). Save the used fish flakes for Second Dashi.
For Second Dashi: Place reserved kelp and bonito flakes with 8 cups cold water in a large saucepan. Bring to a boil, reduce heat to low, and simmer for 30 minutes. Strain broth through a cheesecloth-lined sieve, pressing down on solids before discarding them.
Makes about 6 cups.
Make Ahead Tip: Cover and refrigerate for up to 2 days or freeze for up to 3 months.