½ cup egg whites (from about 4 eggs)
or
½ cup aquafaba (drained from a 15.5-ounce can of chickpeas)
14 tablespoons (one cup minus 2 tablespoons) granulated sugar
1/4 teaspoon cream of tartar
Pinch of kosher salt (skip if beans are salted)
½ teaspoon pure vanilla extract
About 1 tablespoon unsweetened cocoa powder
4 ounces bittersweet chocolate, broken up
1. Prep: Heat oven to 200 degrees. Line two rimmed baking sheets with parchment paper or silicone mats.
2. Whip: Pour egg whites or aquafaba into the bowl of a stand mixer. Whisk in cream of tartar and salt (if using). Using the whisk attachment, whip to soft peaks, about 3 minutes. Turn off mixer.
3. Boil: Measure sugar in a very small saucepan. Stir in about ¼ cup water—enough to create a liquid. Set pan over medium-high heat and cook (no stirring!) until syrup reaches the soft-ball stage, 240 F, about 3 minutes. Pull pan off heat.
4. Whip: Flick mixer to medium-high speed. Mindful of spatter, slowly pour in hot syrup. Continue whipping until bowl is cool and meringue is glossy and sturdy, about 3 minutes. Whip in vanilla extract.
5. Pipe: Scrape meringue into a pastry bag fitting with a plain ½-inch tip. Pipe domes (about 1½ -inches in diameter) for caps and (separately) short columns for stems.
6. Neaten: If your domes sport points on top (mine always do), dip a finger in cool water and pat gently. You may also want to flatten the tops of the stems to create a smooth surface for later gluing stems to caps. Dust a few flecks of cocoa over caps.
7. Bake: Slide pans into the oven and bake until crisp, about 2 hours. Let cool.
8. Glue: Zap chocolate to melt, in two 1-minute bursts. Stir smooth. Using a small pastry brush, paint the flat surface of one dome with chocolate. Press on a stem. Set aside to dry. (A bowl of granulated sugar makes a good spot for drying. Nestle in mushrooms, rounded caps down, stems up, until set.) Repeat with remaining caps and stems.
