For the cold brew method, cover the kombu and shiitake mushrooms in water in a large container and soak them overnight (at least 8 hours) or for the best results, 24 hours. For the hot brew method, heat Water (1 liter) in a medium-sized saucepan on high heat until it starts to boil. Add the Kombu (10 gram) and reduce to medium-low heat. Cover the pan with a lid and simmer for 10 minutes.
Drop in the Dried Shiitake Mushroom (10 gram) and cook for another 10 minutes.
Turn off the heat but keep the lid on and let the mixture rest for 10 minutes.
Once you’ve made the dashi, either from the cold or hot brew method, strain it through a cheesecloth or fine mesh.