Hoshi-shiitake / dried shiitake mushroom

Select hoshi-shiitake with a thick umbrella. Many inexpensive products have thin umbrellas, and some of them are made to look fatter by rolling the edge inward.

Soak in water for a few hours (warm water expedites the process) or microwave if in a hurry.
When microwaving, use a large- or tall-enough container and add water to the shiitake height, and start with 30-60 seconds. Repeat as necessary. Check frequently, as the water tends to boil over. Dilute the liquid as necessary when using it as dashi stock.