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Preheat the oven to 200°F and place the oven rack in the center of the oven. Line a baking sheet with parchment paper.
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In a bowl, beat the egg white and cream of tartar with an electric mixer until soft peaks form.
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Once they form soft peaks, increase the speed to high and gradually add the sugar, a tablespoon at a time and beat until the peaks are very stiff.
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Place the meringue into a large pastry bag fitted with a ½-inch round tip.
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For the caps, pipe 1 - 1½-inch rounds onto the prepared baking sheet, leaving an inch between the rounds.
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For the stems, hold the pastry bag perpendicular to the baking sheet. Start piping gently, forming a stem's base, then lift the bag while squeezing. Make sure to make the pointed end by pulling up the bag. The stems should be about 1 inch high.
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Pipe a few stems extra since some stems can collapse.
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Dip your finger in a little water and gently smooth the tops of the caps.
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Bake for 2 hours until crisp and completely dry.
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Shut off the heat and allow the meringues to cool inside the oven until they reach room temperature.
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Once the meringues have cooled, dust them with cocoa powder, then use your fingers to rub the cocoa powder into the caps and stems until each has darkened to a rustic brown.
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Assembly: melt the chocolate in the microwave or a double boiler. While the chocolate is cooling, make a hole on the bottom of each cap with the tip of a knife. Using a serrated knife, carefully trim about ¼ of an inch off the pointed tips of the mushroom stems.
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Spread a generous coat of melted chocolate over the flat side of several mushroom caps, then attach the cut surface of the stems. Repeat until all of the mushrooms are assembled.
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To prevent them from becoming sticky, put them in an airtight container as soon as you make them.