Position racks at the top and bottom thirds of the oven. Preheat to 300°F (150°C). Line 2 baking sheets with parchment paper.
Whisk together the confectioners' sugar, ground almonds and cocoa. In the large bowl of a stand mixer, beat the egg whites on medium speed until soft peaks form. Increase the speed to medium-high and beat in the sugar one tablespoon at a time, continuing until stiff peaks form. Beat in the vanilla, then use a spatula to gently fold in the almond mixture and the chopped chocolate.
Drop the meringues by tablespoonfuls onto the prepared sheets (or, if you're feeling fancypants, pipe them into pretty "kisses" with a pastry bag). Bake for 10 minutes, then reduce the oven temperature to 200°F (95°C) and bake 1 hour more. Cool to room temperature before removing from the pans. Store in a cool, dry place.
Notes
Room-temp whites: They whip to greater volume; make sure the bowl is completely grease-free.Add sugar slowly: One tablespoon at a time builds a stable, glossy meringue with stiff peaks.Low and slow: The long bake at 200°F dries them to a crisp, melt-in-your-mouth texture.Keep them dry: Store in a cool, dry place; humidity makes meringues weep and soften.