Heat the oven to 375°F and line a baking sheet with parchment paper. Whisk the powdered sugar, cocoa powder, and salt with a fork in a medium bowl to combine. Whisk together the egg whites and the vanilla with the fork in a small bowl.
Make a well in the center of the dry ingredients and pour in the egg whites; stir the mixture with the fork until it just begins to come together. Add the chocolates and stir until well combined. The dough will be extremely sticky and as dark as black licorice.
At the bakery, we use a #60 scoop (like a small ice cream scoop) to scoop and ball these, but an ordinary 1 tablespoon measuring spoon works well too. Pack the batter into the spoon by squashing and dragging the spoon against the inside of the bowl to make sure the rounds of dough are tight and compact—if the dough is too loosely packed, the cookies tend to really spread out and separate as they bake.
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. When the cookies are done, they will be quite gooey, but they will continue to cook as they cool. Once they’ve cooled off enough to eat, they should be soft and chewy—if they’re hard or crisp, they’ve baked too much.
Cool the cookies on the paper, set on a wire rack, for 10 minutes. Repeat with the remaining dough. Serve these cookies the day they are made.