Preheat the oven to 200°F and line a baking sheet with parchment paper.
Add 3 large egg whites to the bowl of a stand mixer and whisk with the whisk attachment on low speed until fluffy and soft peaks form. You can also use a mixing bowl with a hand mixer.
Slowly add the maple syrup and cream of tartar to the whipped egg whites. Increase the speed to medium-high and whisk until stiff peaks form, about 5 minutes. Add the cocoa powder and whisk for another 30 seconds until thoroughly blended.
Put the mixture into a piping bag with a star tip.
Pipe about 1 tablespoon of the meringue onto the cookie sheet for each chocolate meringue cookie. If you don't have a piping bag you can spoon a moundful of the mixture onto the tray.
Bake for 90 minutes to 2 hours, or until the meringues start to turn golden brown and are crispy on the outside.
Remove from the oven and let them cool for 5-10 minutes before removing them from the tray.
Notes
Store any leftovers in an airtight container at room temperature.