Preheat oven to 250F. Line 2 baking sheets with parchment paper and set aside.
Using a handheld mixer or a stand mixer fitted with the whisk attachment, whisk 5 egg whites, ½ teaspoon cream of tartar and a pinch of salt on medium speed until soft peaks (6-8 minutes).
Switch to low speed and very slowly (I do it one teaspoon at a time) add the 1 cup of sugar until your meringue is stiff, sticky and glossy (5-6 minutes).
Sprinkle the cocoa powder on top of the meringue and gently fold until combined. Sprinkle the chopped chocolate on top of that and fold 2-3 times just until the chocolate is spread out (you don't want to fully incorporate, you want to see chunks of chocolate in places).
Drop meringue onto your prepared pans by the tablespoonful and gently flatten and swirl the top with a spatula or the back of a spoon.
Bake for 10 minutes (position your pans in the middle of your oven - for me this means placing them in the top and lower third racks) then turn the oven off. Leave the pans in the oven for at least 6 hours or overnight. Do not open the oven door so the meringues can dry properly.
Remove from the pan and store in an airtight container.