Add the egg whites, salt, and sugar into the bowl of your stand mixer. Beat on med-high for 15-20 minutes. Don't worry about over-beating; as long as you aren't on too high, you pretty much can't.
Draw an eight-inch diameter circle in parchment paper (I traced a plate) and flip it over to line your baking sheet.
Preheat your oven to 350°F.
Once your egg whites have been beaten to shiny, stiff peaks, sift in the corn starch and cream of tartar, and fold it to mix with the rubber spatula.
Sift in the cocoa powder and fold to mix; it should turn a pale brown color. Make sure you get all the powder from the bottom of the bowl.
Add your chopped chocolate pieces and fold to mix them in.
Using the spatula, scrape and spread the meringue onto the circle on your parchment paper. It will spread out a bit as it bakes. Try to form a slight depression in the center, to make a bowl shape.
Put the meringue into the oven and immediately reduce the heat to 225°F. The initial heat crisps the outside, while the lower heat slowly penetrates and bakes the inside.
Bake for an hour. You may want to bake it a little longer if the inside is still the texture of whipped cream; ideally it will be a little gooier.
When your baking time is up, prop open your oven door slightly with a wooden spoon or similar heat-resistant object, and let cool slowly with the meringue inside.