Crack your eggs one at a time, carefully separating them, and pouring the whites into your mixing bowl. I recommend cracking them into a separate bowl and pouring that into the mixer, just in case a yolk breaks (so you only lose one egg, not all of them.)
Add your sugar and pinch of salt to the mixing bowl and start to whip with the whisk attachment on medium-high. Don't go too high.
Preheat your oven to 225°F Line your baking sheets with parchment paper.
Whip your egg whites until stiff peaks form and the mixture is fluffy and shiny; about 10-15 minutes.
Sift your corn starch and cream of tartar into the meringue, and fold carefully to mix.
Sift in your cocoa powder, and add your chopped chocolate, again folding to mix.
Scoop your meringue into a piping bag. Carefully pipe out your cookies, to the size/shape you want. Leave a small amount of space between them, but they shouldn't spread. This recipe makes about 40-50 cookies, depending on size, so you may need more than one batch.
Bake for 90 minutes. Then, turn off the heat and let them cool with the oven door closed, so they cool gradually. Rapid cooling will make them crack (like in my picture) but doesn't affect the texture or taste. Enjoy when fully cooled, usually 2-3 hours.