From Maida Heatter's Book of Great Desserts
Adjust two racks to divide the oven into thirds. Preheat oven to 225 degrees. Cut aluminum foil to fit two cookie sheets at least 12 x 15 inches.
In the small bowl of electric mixer at moderately slow speed, beat the whites for about half a minute or until they are just foamy. Add the salt and cream of tartar. Increase the speed to moderate and beat for almost a minute more until whites hold a soft shape. Continue to beat and start adding the sugar, 1 rounded tablespoonful at a time - beat about half a minute between additions. When half of the sugar has been added, add the vanilla and then continue adding the sugar as before. When all of the sugar has been added, increase the speed to high and beat for 7 to 8 minutes more or until the meringue is very stiff and the sugar is dissolved - test it by rubbing a bit between your fingers - it will no longer feel gritty. (Total beating time from start to finish is about 15 to 18 minutes, but it depends on the power of your mixer.)
To hold the aluminum foil in place, put a dot of the meringue in each corner of the cookie sheets. Cover with the foil and press firmly on the corners.
Do not let the meringue stand. Fit a large pastry bag (preferably at least 15 - 16 inches long) with a plain, round tube 1/2 to 3/4 inch in diameter. (I like to use one that is 5/8 inch.) Fold down the top of the bag to form a deep cuff on the outside. Support the bag by holding it under the cuff with one hand. Using a rubber spatula, with your other hand, transfer all of the neringue into the bag. Lift the cuff up and twist the top closed.
On one piece of the prepared aluminum foil, shape the mushroom stems first. Hold the bag at a right angle and 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. Don't worry if a small point is left on top of the stem; it can be removed later on. The stems may be about 1 to 1-3/4 inches high (the taller they are, the more difficult), but they may vary as real mushroom stems do. They should be place 1/2 to 1 inch apart on the foil. (some of the stems may fall over on their sides, so it is a good idea to make a few extras to be sure you wind up with a stem for each cap.)
Strain the cocoa through a fine strainer lightly over the stems to imitate soil and natural mushroom coloring. Place in the oven on the higher rack.
On the other piece of foil, shape the mushroom caps. Holding the bag straight up and 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 inches 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. The top should be as smooth as possible. You may smooth them out with a dampened knife.
Strain the cocoa lightly over the caps. Bake on the lower rack.
The measurements I have given are approximate - don't worry about them. Smaller or larger mushrooms are equally attractive. Even mushroom meringues with crooked stems or with a slight point on the cap will look great when finished.
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 - and the better - but they should not be allowed to color (it affects the taste). 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.
Remove meringues from the foil. They may be placed on a clean piece of foil, on wax paper, or on a tray. Immediately, while the meringues are very crisp, using a finely serrated knife or a sharp paring knife, gently saw any points off the tops of the stems cutting parallel with the base.
One ounce of chocolate will be needeed for 5 mushroom caps if they measure 1-3/4 to 2 inches in diameter. Using this formula, figure how much chocolate you will need and cut it coarsely and place in the top of a small double boiler over warm water to melt slowly over low heat. When almost melted, remove from the heat and stir until completely melted and smooth.
Hold a mushroom cap upside down. With a demitasse spoon, spread a layer of chocolate over the bottom of the cap, spreading it just to the edge. It should be thin, but not too thin. Place a stem upside down on the chocolate.
Now the mushroom must stand in that position, upside down, until the chocolate hardens. There are several ways to do this. The inverted mushrooms will rest securely in small cordial glasses, small brandy snifters, small egg cups or in an empty egg carton - it will depend on their size.
Carefully place the mushrooms in their cordial glasses or egg cups or whatever in the freezer or refrigerator _only_ until the chocolate is firm. Do not freeze or refrigerate any longer. (Do not freeze mushrooms after the chocolate has hardened - it will cause the finished mushrooms to come apart.) Remove and store at room temperature.
Do not cover the mushrooms airtight. I have kept them for weeks in an open straw basket in an air-conditioned room. They become drier, crisper, and better.
Serve the mushrooms either standing upright on a platter, or tumbled in a basket like real mushrooms, which these will resemble to an unbelievable degree. (Try a napkin-lined basketful as a centerpeice - at dessert time pass it aroiund.) These may be eaten any way, but I suggest upside down, stem first.
The number of mushrooms this recipe yields will depend on their size - approximately 24 rather large or 36 medium. If you want more, prepare and bake one batch, and then repeat; the meringue should not stand any longer than necessary before baking.
Notes: almost any chocolate may be used for this recipe. My first choice is any commercial coating chocolate because it will never discolor and the mushrooms may be kept for weeks or even months. (Commercial coating chocolate will harden quickly and it will not be necessary to chill the mushrooms in order to set the chocolate.) Other chocolates are liable to discolor after a day or two. However, if the meringues are to be served rather soon, I have had good results with Baker's Semisweet, Baker's German Sweet, and Hershey's Special Dark Chocolate.
Straining the cocoa over the mushrooms is easily accomplished by using a tiny wire mesh tea ball.
I just bought a 5/8" pastry tip [# 1A] which works great for the caps, but is really a bit large for stems for little'ish mushies; if I were to use the big tip, I'd make the caps big, or mix tips [a hassle!] for caps vs stems.
Something that Just Happened Yule 1999 - extra mushroom cap, sawed in half with a sharp knife [be prepared for the discard-half to crumble] became a really cool bracket / shelf fungus on the Yule log, pasted into place with a little extra frosting!