1Heat the oven to 200°F and arrange the racks to divide the oven into thirds. Line 2 baking sheets with parchment paper; set aside.
2Place the egg whites and cream of tartar in the clean, dry bowl of a stand mixer fitted with the whisk attachment and whisk on medium speed until soft peaks form.
3Increase the speed to high, gradually add 1/2 cup of the sugar about a tablespoon at a time, and whisk until stiff peaks form.
4Remove the bowl from the mixer. Using a rubber spatula, fold in the remaining 1/4 cup of sugar. Transfer the meringue to a pastry bag fitted with a 1/2-inch round tip.
5Pipe the stems and caps of the mushrooms: Pipe half of the meringue into pointed “kisses” about 1 inch high to make the stems on one of the prepared baking sheets. (Don’t worry if the tips bend over or sag.) Pipe the remaining meringue into domes to make mushroom caps on the second baking sheet (make sure you have an equal number of stems and caps).
6Place the cocoa powder in a fine-mesh strainer and lightly dust it over the stems and caps. Fan or blow on the cocoa powder vigorously to blur it and give the mushrooms a realistic look.
7Place both baking sheets in the oven and bake until the meringues are crisp and completely dry, rotating the sheets from top to bottom and front to back halfway through, about 2 hours. Remove the baking sheets to wire racks and let them cool completely. If you’re not assembling the mushrooms immediately, store the cooled caps and stems in an airtight container to prevent them from becoming moist and sticky.
8To assemble the mushrooms: Place the chocolate in a small bowl set directly in a wide skillet of barely simmering water. Immediately turn off the heat and stir the chocolate until it’s melted and smooth.
9Use a sharp knife to cut 1/4 inch to 1/2 inch from the tip of each stem to create a flat surface. Using a knife, generously coat the flat side of several mushroom caps with melted chocolate. Allow the chocolate to set partially, about 2 minutes. Attach the flat cut surface of the stems to the chocolate and repeat with the remaining stems and caps. Set the assembled mushrooms aside until the chocolate has hardened, the caps and stems are firmly attached, and the mushrooms have completely cooled. Store the mushrooms at room temperature in an airtight container for up to 4 weeks.
UPGRADES Coffee Meringue Mushrooms: Stir 1 1/2 teaspoons instant coffee or espresso powder into the portion of sugar that is gradually beaten into the egg whites.
Chestnut Meringue Mushrooms: Mix 3 tablespoons chestnut flour with the 1/4 cup of the sugar that is folded in at the end.