Preheat oven to 350° F and line two cookie sheets with parchment paper.
In a large mixing bowl, mix cocoa powder, powdered sugar and fine sea salt.
Add in egg whites and vanilla and whisk until mostly smooth but don't over-whisk.
Using a small cookie scoop, fill just over half full and drop onto cookie sheets, at least two inches apart as these will spread.
Sprinkle a few sea salt flakes on each cookie before placing in oven.
Bake cookies for 11-13 minutes. They are done when the sides are firm to the touch and the tops are glossy and cracked.
Slide the parchment paper off of the cookie sheet and then gently loosen and slide the cookies from parchment paper. Let cool.