How to Make It
Place water and kombu in a medium saucepan over high; heat until steaming. Remove from heat; remove and discard kombu.
Sprinkle katsuobushi over broth; steep 3 minutes. Strain through a double layer of cheesecloth; discard solids.
Allow any unused stock to cool to room temperature, then store, tightly covered, in refrigerator. Dashi will keep up to 5 days, but it loses its delicate smoky flavor if frozen. Cloudy sediment at the bottom of the container signals spoilage.