Are the stems of rehydrated dried Shiitake usable

I'll elaborate on Joe's answer some. It's a little known fact that button mushrooms (also called common, white, or table mushrooms), cremini (crimini, baby portobello, baby bella, brown mushroom), and portbello (portabella) are all the same species. Specifically, agaricus bisporus. The difference in appearance and taste is based solely on maturity. The youngest are white button mushrooms, and the oldest are the big portobello mushrooms. There can be slight differences in skin color for the portobello's, though it tends to be darker than the stark white of the button.

The mushrooms in that video are most certainly fully mature agaricus bisporus, a portobello.

It's a safe bet that anytime you see a mushroom being substituted for a "steak", that it will be a portobello. They have a rich meaty flavor and texture that is perfectly suited for this.

I answered a question like this previously. How to work with Dried Mushrooms?

Cold water - the best way to keep the original flavour.

It will take at least couple of hours.

1. I would soak them in cold water for about 15 mins

2. Pour all the water out. Put in fresh water and keep soaking it. I will do this step couple of times. Remember you may retain the water as flavour, but you have to clean the mushroom and make sure the bitterness is gone. Also, I must say the imported Chinese mushroom must be handled carefully as there have been reports their process may contain some chemicals.

3. After a couple of hours, make sure the mushrooms are soft and take them out of water and drive them thoroughly. You may keep the water if you wish. The mushrooms are good for steamed and stir-fry.

Hot water

If you are in rush, then you may use hot or warm water. You still have to go through the cleaning process, but the time required will be less.