Preheat oven to 350 degrees F. Line two rimmed baking sheets with parchment paper. Spray the parchment paper lightly with cooking spray.
In a large bowl, whisk together the powdered sugar, cocoa powder, and salt. Beat in two egg whites and the vanilla extract 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. Fold in chocolate chips. Let the batter rest for 15 minutes.
Spoon batter onto the prepared baking sheets in 12 evenly spaced mounds per cookie sheet. Bake for 12-14 minutes, until the tops are glossy and lightly cracked. Slide the parchment paper (with the cookies) onto wire racks. Cool completely. Enjoy!
Make it DAIRY-FREE: Use dairy-free chocolate chips like Enjoy Life Foods to keep these cookies dairy-free.
The 15 minute rest helps these cookies have the perfect texture and consistency.