Preheat the oven to 225°F. Line two baking sheets with parchment paper.
Beat the egg whites in a clean, dry mixing bowl on medium speed until foamy, about 1-2 minutes.
Add the cream of tartar and salt. Continue beating until soft peaks form, about 3-4 minutes on medium speed.
Increase the mixer to high speed. Gradually add the sugar, approximately 1 tablespoon at a time. Beat until the meringue is thick, glossy, and holds stiff peaks, about 3-5 minutes. Mix in the vanilla extract.
Sift the cocoa powder over the meringue to prevent lumps. Fold it in gently with a spatula just until combined. Do not continue mixing once the cocoa disappears.
Pipe or spoon the meringue into dollops onto the prepared baking sheets.
Bake for 45 minutes, until the cookies feel dry and lift easily from the parchment.
Turn off the oven and leave the cookies inside with the door closed for 1 hour to finish cooling.