Preheat the oven to 225º F.

Line two baking sheets with parchment or wax paper.

Whisk egg whites, sugar, cream of tartar and salt in a large heatproof bowl set over a saucepan of simmering water. Do not let the bottom of the bowl touch the simmering water, there should be about an inch of space between the two. Whisk the egg white mixture until the sugar has dissolved and the whites are very frothy. Remove the bowl from the heat and continue to whisk the eggs with a hand held mixer or a standing mixer until stiff, glossy peaks form and the eggs have cooled, about 5 minutes.

To make the mushrooms, spoon the meringue into a large pastry bag fitted with a plain tip. Pipe half of the meringue onto the prepared baking sheet to form 2-inch domes. Pipe the remaining meringue into stem shapes for each dome.

Sprinkle cocoa powder lightly over the meringues.

Bake unitl dry, about 2 hours.

To form the mushrooms, use a small pairing knife to make a small hole on the underside of each mushroom cap. Spread chocolote on the underside of the mushroom and gently attach the stems. Rest the mushrooms upside down in an empty egg carton until the chocolate has cooled.

Store in an airtight container until ready to use.