Preheat the oven to 225F.
Line a large sheet pan with parchment paper. Set aside.
Add the egg whites to the bowl of a stand mixer fitted with the whisk attachment. Beat on medium-high speed (Kitchenaid #8) until the egg whites become foamy.
Add the cream of tartar, if using.
Next, with the mixer still on medium-high speed, slowly add the sugar about one tablespoon at a time. Adding all of the sugar should take about two minutes.
Beat the egg whites until stiff peaks form.
Once the egg whites have reached stiff peaks, gently fold in the cocoa powder. Be very gentle, making sure to not deflate the egg whites!
Carefully transfer the mixture to a piping bag fitted with a large star, large round, or large open star attachments. Pipe small dollops (about 1” in diameter) that are at least 1" inch apart.
Bake the meringues for exactly one hour, and then turn off the oven and slightly open the oven door but leave the meringues inside! Allow them to slowly cool to room temperature inside the warm but cooling oven.
Once the meringues have cooled, place them in an airtight container, and store them at room temperature for up to three days.