Preheat the oven to 350°F and line 2-4 rimmed half sheet pans with parchment paper.
Sift the powdered sugar and cocoa powder into a large bowl and whisk to combine.
Stir in the egg whites, vanilla extract, and salt. Mix until all the sugar/cocoa is moistened. Then stir in the chocolate chips.
Using a medium ice cream scoop (1½ tablespoons), scoop the cookie batter onto 2 of the prepared baking sheets, leaving plenty of room between cookies for spreading. I recommend no more than 8 cookies per baking sheet.
Bake the cookies on the center and lower rack for about 14 minutes, rotating halfway through. The cookies are done when the tops are glossy and crackly and the edges are set. Repeat with the remaining cookie dough on the remaining sheet pans, as needed.
Cool the cookies completely on the baking pans on a wire rack.
Store the cookies in an airtight container for up to 3 days.
• Start with 3 egg whites and if the batter is too thick, add a 4th egg white. The "cookie dough" should be on the thin side and resemble brownie batter.
• If you have to reuse your baking pans, cool them completely before baking again. You can do this quickly by sticking them in the refrigerator.