2 ounces bittersweet or semisweet chocolate, cut into small pieces
Cookie sheet lined with parchment paper Pastry bag fitted with plain 1/2-inch tip
In a large heatproof bowl, whisk the egg whites, cream of tartar, and sugar together until all of the sugar is moistened. Whisking constantly, set the bowl on direct heat or in a pan of simmering water, until the egg whites are warm to the touch. Remove from the heat and beat at high speed until mixture is very stiff and dull looking.
Scrape meringue mixture into pastry bag. Pipe round, button shapes to make mushroom "caps". Pipe pointed "kiss" shapes about one inch tall to make "stems". Dust with cocoa. Bake two hours in a 200 degree oven until crisp and completely dry.
Assemble the mushrooms: Place chocolate in a small bowl. Set bowl in a saucepan of barely simmering water. Immediately turn off heat and stir chocolate until melted and smooth.
Spread a little melted chocolate on the flat side of each meringue mushroom "cap". Use a sharp knife to cut off and discard the pointed ends of meringue "stems". Attach stems to caps while chocolate is still soft. Set assembles mushrooms aside until the chocolate has dried and caps and stems are "glued" together. (Meringue mushrooms may be made 3 to 4 weeks in advance and stored in an airtight container.)