Prepare the Dashi Broth: Gently wipe the kombu with a damp cloth. Place kombu and 3 cups of water in a pot and heat over medium-low heat until just before boiling (small bubbles will form around the edges). Remove the kombu. Add the bonito flakes, bring the liquid to a gentle simmer, then turn off the heat and let steep for 10 minutes. Strain the broth through a fine-mesh sieve lined with cheesecloth to remove all solids. Keep the dashi broth warm.