Put the kombu in a food processor and process it until powdered. Do the same with the dried shiitake mushrooms (make a large amount and keep it in stock for later use).
In a large container, combine the soy sauce, sake, and bonito flakes, put the kombu and shiitake powder in a paper tea filter, then let it sit in the soy sauce overnight.
After letting it sit overnight, bring it to boil on medium heat, then cool. Store in the refrigerator after it cools.
Story Behind this Recipe
At first, I made this with kombu and dried shiitake mushrooms without grinding them into powder, but I realized that it's more efficient and economical to grind it into powder so I do so now.
If you make a stock of this, it is convenient for many uses, such as stewed dishes, takikomi-gohan (rice cooked with other ingredients), sunomono (vinegar-marinated dishes), or tamagoyaki). With 20 g of kombu, you can produce about 1 and 1/2 tablespoons of powder. For dried shiitake, 5 mushrooms will produce about 3 tablespoons.