In a large heatproof bowl, whisk the egg whites, cream of tartar, and sugar together until all of the sugar is moistened.

Set the bowl on direct heat or in a pan of simmering water, whisking constantly, until the egg whites are warm to the touch.

Remove from the heat and beat at high speed until the mixture is very stiff and dull looking.

Scrape the meringue mixture into a pastry bag fitted with a plain 1/2-inch tip.

Pipe round, button shapes onto a cookie sheet lined with parchment paper to make mushroom caps.

Pipe pointed kiss shapes about one inch tall onto the same sheet to make mushroom stems.

Dust the mushroom caps and stems with cocoa powder.

Bake in a 200-degree oven for two hours, or until the meringues are crisp and completely dry.

Place bittersweet or semisweet chocolate pieces in a small bowl.

Set the bowl in a saucepan of barely simmering water.

Turn off the heat immediately and stir the chocolate until it is 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 the stems to the caps while the chocolate is still soft.

Set the assembled mushrooms aside until the chocolate has dried and the caps and stems are glued together.

Meringue mushrooms may be made 3 to 4 weeks in advance and stored in an airtight container.