Preheat the oven to 215°F (100°C) and place a rack in the middle. Line a baking sheet with parchment paper and set aside.
Using a stand mixer or a handheld electric mixer with a large bowl, beat egg whites on medium speed until they start to get foamy.
Add cream of tartar (or vinegar), salt, and vanilla extract and beat mixture again until soft peaks form.
Add sugar — 2 tablespoons at a time — until stiff, glossy peaks form.
Add 1 tablespoon of cocoa powder, and continue to whip the meringue for about 2 minutes. The mixture should be glossy and firm — it should be spoonable, but will keep the shape when dolloped onto a baking sheet.
Spoon batter onto the lined baking sheet. Use an ice-cream scoop for equal portions, or pipe the meringue.
Bake for 2 hours, rotating the baking sheet halfway through. Adjust cooking time based on the size of the meringues.
The meringues should feel dry and very light, and come off of the parchment paper easily.
Let cool completely on the baking sheet before removing.
Once cooled, dust the meringues with the remaining tablespoon of cocoa powder. Keep in an airtight container for up to 2 weeks.
