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.