Meringue Mushrooms

Preheat oven to 225°F. Beat egg whites with an electric mixer fitted with whisk attachment on medium speed until foamy, about 30 seconds. Add cream of tartar, and continue beating on medium until soft peaks form, about 1 minute. With mixer running, gradually add sugar and salt, beating until completely incorporated, stiff peaks form, and mixture is glossy, 3 to 4 minutes. Whisk in vanilla by hand. 

Transfer egg white mixture to a piping bag fitted with a 1/2-inch round tip. Pipe about 30 small cones of varying sizes, 1/2- to 1-inch tall, on a parchment paper-lined baking sheet, spacing at least 1/2-inch apart. These will be used as mushroom stems. Pipe about 30 small circles of varying sizes, 1/2- to 1 1/2-inches wide, on prepared baking sheet, spacing at least 1/2-inch apart. These will be used as mushroom caps. Discard any leftover egg white mixture. Dust circles lightly with unsweetened cocoa.

Bake in preheated oven until meringues are dry, 1 1/2 to 2 hours. Cool completely on baking sheet on wire rack, about 30 minutes.

Working with 1 mushroom cap at a time, twist tip of a small sharp knife in center of flat side of mushroom cap to carve a 1/4-inch hole. Spread about 1/4 teaspoon of melted chocolate in and around hole to resemble mushroom gills and to act as glue to hold stem. Press pointed end of 1 mushroom stem into hole. Place mushroom, cap side down, on a rimmed baking sheet. Repeat with remaining meringues and chocolate. Let mushrooms stand at room temperature until chocolate hardens, about 1 hour. Mushrooms can be stored in an airtight container at room temperature for up to 3 days.