How to Make It
Combine 6 cups water, shiitakes, and kombu in a 3-quart classic-style donabe. Let stand for 30 minutes.
Heat mixture, uncovered, over medium-low until steaming but not simmering (about 140°F), about 10 minutes. Cook 15 minutes, adjusting heat as needed to prevent simmering. Using a slotted spoon or spider, remove and discard kombu.
Stir in dried smoked bonito flakes, if using, and continue cooking over medium-low until flavors are infused, dashi has a light smoky flavor, and liquid has reduced to about 4 cups, 12 to 15 minutes. Remove from heat; pour through a fine wire-mesh strainer into a bowl, reserving shiitakes for quick-pickled shiitakes.