Adjust 2 racks to divide the oven into thirds.  Preheat oven to 225.  Cut aluminum foil to fit 2 cookie sheets.

In small bowl beat egg whites at slow speed until they are just foamy. Add the salt and cream of tartar.  Increase the speed to medium and beat for 1 minute more until whites hold a soft shape.  Continue to beat and start adding sugar, 1 rounded TBSP at a time, beat about 1/2 minute between each addition.  When half of the sugar has been added, add the vanilla and then continue adding the sugar.  When all of the sugar has been added, increase the speed to high and beat for 7-8 minutes more or until the meringue is very stiff and the sugar is dissolved (test by rubbing between fingers)

Fit a large pastry bag with a plain, round tube (1/2 to 3/4 inch in diameter).  Or if you are using disposable bags you can cut the hole.  Fill with meringue.

On one piece of aluminum foil shape the mushroom stems first.  Hold the bag at a right angle, close to the foil.  Press the meringue out gently while slowly raising the bag straight up.  The base of the stem should be a bit wider for support.  Keep the stem as straight as possible.  Hold the bag upright and steady with one hand while, with the other hand, use a small knife to cut the meringue away from the tube.  The stems may be about 1 to 1 3/4 inches high, but they may vary as real mushroom stems do.  They should be paced 1/2 to 1 inch apart on the foil.  (some of the stems WILL fall, so it is a good idea to make a few more than you need.)  Sift cocoa lightly over the stems to imitate soil and natural mushroom colouring.  Place in the oven on the higher rack.

On the other piece of foil, shape the mushroom caps.  Holding the bag straight up, close to the foil, press out even rounds of the meringue.  The caps should be placed about 1/2 inch apart.  The caps may average about 1 1/2 to 1 3/4 inches in width and 3/4 inch in height, but they may also vary as real mushroom caps do.  Sharply twist the bag away to avoid leaving a peak on the top.  (A wet, but not dripping, fingertip will help to smooth out any peaks)  Sift cocoa lightly over caps.  Bake on the lower rack.

Bake for 1 hour, or a bit longer depending on size, until meringues may be lifted easily from the foil and the bottoms are firm to the touch.  The longer they bake the drier they are--but they should not be allowed to colour.  Turn the heat off, prop the oven door open a little, and let the meringues dry out even more in the turned-off oven until cool - about another hour.

Remove meringues from the foil. They can be placed on a clean piece of foil or wax paper.

Melt the chocolate in either a double boiler or according to microwave directions (on package).  Glue the caps to the stems by coating the bottom of the caps with a layer of melted chocolate.  Then attach stem (trim off pointed ends with a knife) and let the mushrooms sit upside down until the chocolate hardens.  Do not freeze/refrigerate mushrooms (the air is too moist), the best method is to actually leave them in a bowl lined with a towel and to cover loosely with the towel.