Wipe the surface of the kombu kelp (10 cm by 7 cm piece) with a tightly wrung wet cloth. (The white powder on the surface is where much of the umami flavor is, so do not wipe it off completely.) Heat a pot of water (2 liters). When you can feel the warmth of the water (about 30°C), add the kombu. When it begins to bubble (about 70°C), add the bonito flakes (35 g). Simmer for about 3 minutes without letting the water come to a boil. Remove from the heat and set aside until the bonito flakes sink to the bottom of the pot. Slowly strain the broth using a paper towel in a mesh sieve without shaking. Let the liquid drain naturally and do not squeeze the bonito flakes.