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1
Pour cold filtered water into a large pot and submerge kombu pieces.
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2
Bring to 60°C over medium heat (approximately 15 minutes); do not boil, as heat above 70°C releases excess iodine and bitterness.
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3
Remove from heat and let steep at 60°C for 8 minutes, allowing umami compounds to infuse.
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4
Strain through fine-mesh strainer lined with cheesecloth, reserving kombu for secondary broth if desired.
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5
Return broth to pot, bring to bare simmer, and add bonito flakes; immediately remove from heat.
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6
Let bonito settle for 4 minutes undisturbed, then gently strain through cheesecloth without pressing.
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7
Use immediately or cool and refrigerate for up to 5 days; freeze in ice-cube trays for 2 months.
