Chocolate Meringue Cookies

Adjust oven racks to upper-middle and lower-middle positions and heat oven to 225 degrees. Line 2 baking sheets with parchment paper. Combine sugar and cornstarch in small bowl.

In stand mixer fitted with whisk attachment, beat egg whites, vanilla, and salt at high speed until very soft peaks start to form (peaks should slowly lose their shape when whisk is removed), 30 to 45 seconds. With mixer running at medium speed, slowly add sugar mixture in steady stream down side of mixer bowl (process should take about 30 seconds). Stop mixer and scrape down sides and bottom of bowl with rubber spatula. Return mixer to high speed and beat until glossy, stiff peaks have formed, 30 to 45 seconds. Gently fold bittersweet chocolate into meringue mixture.

Working quickly, place meringue in pastry bag fitted with 1/2-inch plain tip or large zipper-lock bag with 1/2 inch of corner cut off (see related content). Pipe meringues into 11/4-inch-wide mounds about 1 inch high on baking sheets, 6 rows of 4 meringues on each sheet. Bake 1 hour, rotating pans front to back and top to bottom halfway through baking. Turn off oven and allow meringues to cool in oven at least 1 hour. Remove from oven and let cool to room temperature before serving, about 10 minutes.