Heat oven to 225 degrees. Line a baking sheet with parchment paper, and set aside.
In a small saucepan, heat sugar and 1/2 cup water over low heat until sugar dissolves. Bring to a boil; cook until liquid reaches 248 degrees.(hard-ball stage) on a candy thermometer.
Meanwhile, in the bowl of an electric beater fitted with the whisk attachment, whip egg whites on low speed until soft peaks form. Increase speed to high, and add hot syrup in a steady stream, beating constantly. Continue beating until cool and stiff, about 5 minutes. Beat in vanilla. Fold in cocoa powder.
Spoon meringue into a large pastry bag fitted with a coupler and large plain tip. Pipe meringue onto prepared baking sheet to form 2-inch domes. Pipe a separate stem shape for each dome.
Sprinkle cocoa powder lightly over meringues. Bake until dry, about 2 hours. Store in an airtight container until ready to use.
To assemble mushrooms, melt chocolate in a double boiler or in a heat-proof bowl set over a pan of simmering water. Trim off points from tops of stems. With a small offset spatula, spread chocolate on underside of a cap and place trimmed end of stem into center of cap. Place mushroom, stem side up, in an egg carton to harden. Repeat with remaining mushrooms; refrigerate until set.
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