3 eggs whites, at room temperature
1/8 t. cream of tartar
3/4 C. sugar, preferably superfine
1 1/2 t. instant coffee or espresso powder (optional)
About 2 t. unsweetened cocoa powder, for dusting
2 oz. bittersweet or semisweet chocolate, cut into small pieces
Baking sheet, lined with parchment paper
Large pastry bag
Plain pastry tip with a 1/2-inch opening
Position a rack in the center of the oven. Preheat the oven to 200°F.
In a clean, dry mixing bowl, beat the egg whites and cream of tartar on medium speed until soft peaks form when the beaters are lifted. On high speed, gradually add the sugar with all the coffee powder, if using, about 1 tablespoon at a time. The mixture should stand in stiff glossy peaks when the beaters are lifted. Scrape the meringue into the pastry bag. Pipe tall pointed "kisses" about 1 inch high to make mushroom stems. Don't worry if the tips bend or sag. Pipe domes to make mushroom caps and stems and fan them or blow on them vigorously to blur the cocoa and give the mushrooms an authentic look. Bake until crisp and completely dry, 2 hours.
To assemble the mushrooms, place the chocolate in a small bowl set in a skillet of barely simmering water. Immediately turn off the heat and stir the chocolate until melted and smooth. Use a sharp knife to cut 1/4 to 1/2 inch off the tip of each stem, to create a flat surface. Spread a generous coating of melted chocolate over the flat side of several mushroom caps. Allow the chocolate to set partially before attaching the cut surface of the stems. Repeat until all the mushrooms are assembled. Set assembled mushrooms aside until the chocolate has hardened and caps and stems are glued together. Meringue mushrooms may be made 3 to 4 weeks in advance and stored at room temperature in a an airtight container.