Separate egg whites into the bowl of the electric stand mixer, and let stand 30 minutes, until they come to room temperature.
Preheat the oven to a very low temperature, between 150° and 170°.
Line 2 baking sheets with parchment paper.
Using the whisk attachment, beat the egg whites at medium speed until they become foamy and white.
Add the salt and cream of tartar, and then once again beat until soft peaks form.
Leave the mixer running and, one tablespoon at a time, add the sugar, waiting 15 to 20 seconds between additions. This step will take about 7 minutes.
Stir the cocoa powder to loosen lumps and then sift it through a fine mesh sieve before carefully folding it into the meringue.
Transfer the mixture to a piping bag fitted with a large round tip or a star tip. Pipe the chocolate meringue onto the prepared baking sheets to form 2-inch diameter cookies. Alternately, use a spoon to scoop the meringue into 2-inch diameter dollops on the baking sheet. Spacing can be close.
Place baking sheets in oven and check cookies for doneness after 2 hours, then periodically until the meringues are crisp, but not browned. Bake time will vary, depending on the oven temperature.
Allow the cookies to cool completely before removing them from parchment paper.
Store at room temperature in an airtight container for 2 weeks.