Wipe the kombu with a dry cloth (don't wash off the white umami powder). Cut a couple of slits in it and soak in the litre of cold water for at least 30 minutes.
Heat the pot gently toward a simmer. Just before it boils, when small bubbles appear around the kombu, remove the kombu.
Bring to a brief boil, add the katsuobushi all at once, then turn off the heat and let the flakes steep for 1-2 minutes.
Strain through a paper-towel-lined sieve set over a bowl. Let it drip naturally without pressing the flakes. Use the clear golden dashi right away, or store.
