Preheat the oven to 110 degrees C. Line 2 baking trays with parchment paper or non stick baking paper.
In a clean, large glass or metal bowl, use an electric mixer to whip egg whites until frothy. Add cream of tartar, salt and vanilla. Continue whipping until the whites have soft peaks. Gradually sprinkle in the sugar so that it does not sink to the bottom, and continue whipping until the mixture holds stiff shiny peaks.
Place a round tip into a pastry bag, and fill the bag half way with the meringue. For the stems, press out a tiny bit of meringue onto one of the trays, then pull the bag straight up. Do not worry about making all of the pieces exactly the same. The mushrooms will look more natural if the pieces are different sizes. If they end up being very pointy, you can "file" them off with a serrated knife once hardened and cooled.
To pipe the mushroom caps: squeeze out round mounds of meringue onto the other baking tray. Pull the bag off to the side to avoid making peaks on the top. Dust the mushroom caps lightly with cocoa using a small sieve.
Bake for 1 hour in the preheated oven, or until the caps are dry enough to easily remove from the baking trays. Set aside to cool completely. Melt the chocolate in a metal bowl over simmering water, or in a glass bowl in the microwave, stirring occasionally until smooth.
Poke a small hole in the bottom of a mushroom cap. Spread chocolate over the bottom of the cap. Dip the tip of a stem in chocolate, and press lightly into the hole. When the chocolate sets, they will hold together. Repeat with remaining pieces. Store at room temperature in a dry place or tin.
Been looking for the perfect recipe for these & I finally found it! Had to make an adjustment to my current piping nozzles for the stems. Did need to refrigerate mine though as 'room temperature' is too warm for the Australian Christmas climate. - 22 Dec 2015