French Meringue Cookies

Preheat oven to 300 degrees.

In the bowl of an electric mixer fitted with the whisk attachment, beat together egg whites and salt on high speed until glossy peaks form. With mixer running, add sugar in three additions, beating until meringue is stiff and glossy. Sprinkle in cornstarch, vinegar, and vanilla; gently fold to combine. Sift cocoa over meringue; gently fold so that streaks of cocoa remain.

Using a 1/2-cup ice cream scoop, drop 6 balls of meringue onto a parchment-lined baking sheet, spacing about 3 inches apart. Bake until puffed and slightly cracked, 30 minutes. Turn off oven and let meringues stand at least 2 hours or up to overnight.