4 small shiitake mushrooms, stemmed and caps very thinly sliced
In a medium bowl, whisk the dashi powder into the warm water until it is dissolved. Let the sediment settle, then pour the clear dashi into a large measuring cup; rinse out the bowl. In the same bowl, using chopsticks, very gently stir the eggs until blended, without incorporating too much air. Stir in the dashi, then strain the mixture back into the measuring cup.
Divide the cashews between 4 shallow 1-cup bowls. Top with the egg mixture and wrap each bowl in plastic. Carefully set the bowls in a steamer basket set over boiling water and steam for 14 minutes or until the custards are set. Immediately transfer the bowls to the refrigerator to chill for at least 3 hours or overnight.
Unwrap the custards. Top with the crabmeat, scallion and shiitake and serve.
Dashi powder is a flavorful dried Japanese stock made with dried bonito fish flakes and konbu seaweed. Look for it at Japanese markets.