Step 1 Wipe the surface of the kombu with a moist towel to clean.
Step 2 In a medium saucepan, combine the kombu with 7 3/4 cups water and over very low heat, allow the kombu to soften as it soaks in the water. Slowly raise the temperature of the water over medium heat so a thermometer inserted reads 140 degrees. Maintain the temperature for one hour. While the kombu is steeping, place the bonito flakes in a medium-size bowl.
Step 3Remove the kombu from the broth, increase the heat to medium-high until a thermometer reads 176 degrees. Turn off the heat and immediately pour the broth over the bonito flakes into the bowl.
Step 4Let the bonito flakes sit in the broth, thoroughly soaked, for 10 seconds. Strain the liquid through a cheesecloth-lined fine-mesh strainer. Let the liquid drain naturally, without pressing down on the solids. Discard the solids and store the dashi in the refrigerator for up to one week. Makes about 5 cups dashi stock.