Preheat oven to 350 degrees F and line two rimmed baking sheets with parchment paper.
In a large bowl, whisk together powdered sugar (I know, it's a lot, but you need it!) with cocoa powder and salt. Whisk in egg whites (start w/ just two) and vanilla extract and beat just until the batter is moistened. You're looking for a brownie-like, thick and fudgy batter consistency. If it seems too thick, add another egg white-- then a 4th one if it still seems too thick. Gently stir in chocolate chips.
Spoon batter onto the prepared baking sheets in 12 evenly spaced mounds per cookie sheet.
Bake about 14 minutes, until the tops are glossy and lightly cracked.
Slide the parchment paper with the cookies onto wire racks and let cookies cool completely. Seriously, please, let the cookies cool completely before you attempt to remove them from the mat or the parchment: they're delicate, so gently peeling the paper away from the cookie at room temperature works best. Store in an airtight container for up to 3 days.