1. Preheat oven to 375F and line a baking sheet with parchment paper.
2. Whisk the powdered sugar, cocoa powder and salt with a fork in a medium bowl to combine.
3. Whisk together the egg whites and the vanilla with the fork in a small bowl.
4. Make a well in the center of the dry ingredients and pour in the egg whites.
5. Stir the mixture with the fork until it just begins to come together.
6. Add the chocolates and stir until well combined. The dough will be extremely sticky and as dark as black licorice.
7. Pack the batter into a 1 tbsp spoon by squashing and dragging the spoon against the inside of the bowl to make sure the rounds of dough are tight and compact.
8. Place the rounds of dough on the parchment-lined cookie sheet a good 3 inches apart and bake for about 12 minutes or until the tops are glossy and set.
9. When the cookies are done, they will be quite gooey, but they will continue to cook as they cool.
10. Cool the cookies on the paper, set on a wire rack for 10 minutes. Repeat with the remaining dough.
