Preheat the oven to 225°F. and line 2 large baking sheets with aluminum foil.

In a large bowl, beat the egg whites until foamy. Add the salt, cream of tartar, and vanilla and continue beating until soft peaks form. Add the sugar 1 tbsp at a time, beating for about 30 seconds between each addition. When all the sugar has been added, beat for an additional 6 minutes. The egg whites should be very stiff, glossy, and extremely smooth, but not dry.

To make the mushrooms, spoon the mixture into 2 pastry bags, fitted with a 1/2-inch round tip for the stems and a 5/8-inch tip for the caps. Holding the 5/8-inch tip close to a baking sheet, press out round tops for the mushroom caps. (Any unwanted points can be smoothed with a finger dipped in cold water.)

For the stems, use the smaller tip. Hold the pastry bag in one hand and a small sharp knife in the other. Press out the stem, allowing 3/4 to 1 inch in height, and then cut off the meringue with the knife. The stem will have a flat end that will adhere more easily to the cap. Lightly dust the stems with cocoa powder.

Reduce the oven temperature to 200°F. and bake the meringues for 1-1/2 to 2 hours, until they are dry to the touch and can be lifted from the foil. Cool completely.

To assemble, with a small spatula spread a thin layer of melted chocolate on the underside of the cap. Dip the flat tip of the stem into the chocolate and affix to the cap. Turn upside down and dry. (Egg cartons turned upside down are excellent racks for this.)