Hello and welcome to DC. IMO, shiitakes don't have anywhere near the depth of flavor that Porcini mushrooms have. You can purchase dried, and sometimes fresh porcini, and mixes of wild. dried mushrooms in most grocery stores. The water used to reconstitute the dried mushrooms has a pronounced mushroom flavor, making it great for use in sauces, and soups. You can substitute other mushroom water. but you have to make sure that it is a strong mushroom flavor. Look for chanterelles, portabella, cremini, oyster, and any member of the bolete family. Shiitake, hen of the woods, puff balls, straw mushrooms, and yellow surfer mushrooms, though tasty. are mild in flavor. and won't give you the u,a,i flavor that your recipe is probably looking for.
If you are a mushroom hunter, the wild filed mushrooms found on lawns, on golf courses, and such, withe the pink gills on the bottom, are have amazing flavor, as do shaggy miens that are available in the fall. This time of year, look for morels.
You can purchase porcine mushroom powder on line, to be used a a seasoning for recipes. Here is an example - https://www.thespicehouse.com/produc...ushroom-powder
I hope your recipe turns oput great. By the way, what are ypu making, if I might ask?
Seeeeeya; Chief Longwind of the North