Preheat oven to 300°F. Position oven racks in the upper and lower thirds of the oven. Line 2 baking sheets with parchment paper.
Sift together: In a medium bowl, sift together confectioners' sugar and cocoa powder.
Whisk egg whites: In a large, clean bowl, whisk egg whites and cream of tartar (and salt, if using) on medium speed with an electric mixer until soft peaks form.
Gradually add sugar: Gradually add granulated sugar, 1 tablespoon at a time, beating on high speed until stiff, glossy peaks form.
Fold in flavorings: Beat in vanilla extract. Gently fold in the cocoa mixture using a rubber spatula until just combined.
Pipe meringues: Pipe meringue by rounded teaspoonfuls onto prepared baking sheets, spacing them about 2 inches apart.
Bake: Bake for 25-30 minutes, or until the outside is dry but the inside is still slightly soft. Let cool completely on baking sheets before gently peeling off the parchment.