Step 1In a medium saucepan, bring the mirin to a boil over high heat. Reduce to a simmer and cook 2 to 3 minutes to cook off some of the alcohol. Stir in the sugar until dissolved, then stir in the soy sauce. Cook just until the kaeshi begins to bubble (be careful that it does not boil), then remove from heat.
Step 2If the kaeshi has a film on top, skim this off with a spoon and discard, or remove it by placing a piece of plastic wrap on top (the film will stick) and discarding the plastic wrap. Cool the kaeshi, then pour into a nonreactive container with a tight-fitting lid and refrigerate overnight before using. This makes 4 1/2 cups kaeshi (more than is needed for the final sauce), which will keep, refrigerated, for several months.
Step 1Place the kombu in a medium saucepan with water to cover. Soak the kombu for 20 minutes, then place the pan over high heat and bring to a boil. Add the bonito flakes and immediately remove from the heat. Set the pan aside until the liquid is cool enough to touch, at least 15 minutes.
Step 2Strain the dashi through a cheesecloth-lined sieve, squeezing until all of the liquid is removed. Pour the dashi into a nonreactive container with a tight-fitting lid. This makes 3 to 4 cups dashi (more than is needed for the final sauce), which will keep, refrigerated, for about 1 week.
Step 1Combine the kaeshi and dashi stock and serve on the side of the zaru tofu.