Heat the oven to 225°F (107°C). Line 2 large baking sheets with parchment paper.
Grab a hand blender fitted with the whisk attachment. In a large, deep bowl, beat the egg whites on low to medium-low until foamy, 2 to 3 minutes.
Add the vanilla and cream of tartar and beat until incorporated. With the whisk running, gradually add the superfine sugar and cocoa powder and turn up the blender to medium speed. Beat until the whites hold soft peaks, 3 to 4 minutes.
Using a large spoon, drop large dollops of the whipped egg whites onto the prepared baking sheets about 1 1/4 inches apart.
Bake in the oven for about an hour, until the meringues are crisp on the outside but not browned. They shouldn't be completely dry, nor too wet in the center.
Remove them from the oven, set the parchment onto racks, and let cool. If you like, dip the tips or one one side of the cooled meringues in melted bittersweet chocolate for an extra touch. The cookies keep in an airtight container for up to 5 days.