Set racks in middle and lower positions; preheat oven to 250 degrees F. Line 2 baking sheets with parchment paper or coat with cooking spray and dust with flour, shaking off excess.Advertisement
Beat egg whites in a large mixing bowl with an electric mixer until soft peaks form. Gradually beat in 1/4 cup sugar, beating until stiff and glossy. Fold in the remaining 1/4 cup sugar and vanilla with a rubber spatula.
Spoon the meringue into a piping bag fitted with a 1/4-inch plain tip. (If using parchment paper, stick it to the baking sheets with a little dab of meringue in each corner.) Holding the pastry bag at a right angle to the baking sheet, pipe meringue into 1 1/2-inch round "caps" on one of the prepared baking sheets. On the other baking sheet, pipe 24 "stems" by holding the pastry bag at a right angle and slowly raising the bag up vertically as you pipe the meringue to make a 1- to 1 1/2-inch tall stem a bit wider at the base than the top. Use a knife in the other hand to cut away the stream of meringue between stems. Dust the caps lightly with cocoa.
Bake the meringues until dry and crisp, rotating the baking sheets midway through baking, 50 to 60 minutes. Transfer the baking sheets to wire racks to cool.
To assemble the mushrooms, trim the top of each stem so it is flat, not pointed. Spread a small amount of melted chocolate on the underside of the caps, then gently press the stems into the chocolate. Set aside to dry with the stem side up.
Make Ahead Tip: Store in an airtight container for up to 1 week.
Tip: To melt chocolate, microwave on Medium for 1 minute. Stir, then continue microwaving on Medium, stirring every 20 seconds, until melted. Or place chocolate in the top of a double boiler over hot, but not boiling, water. Stir until melted.