Preheat oven to 350° F and line two 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.
3 cups powdered sugar, 2/3 cup unsweetened cocoa powder, 1/4 teaspoon salt
Whisk in egg whites (start with just two) and vanilla extract and stir just until the batter is moistened. You're looking for a brownie-like, thick and fudgy, batter consistency. If the batter seems too thick, add another egg white, then a 4th one if it still seems too thick.
2 to 4 large egg whites, 1 tablespoon vanilla extract
Stir in the chocolate chips.
1 1/2 cups bittersweet chocolate chips
Spoon batter onto the prepared baking sheets in 12 evenly spaced mounds per cookie sheet.
Bake for about 14 minutes, until the tops are glossy and lightly cracked.
Slide the parchment paper with the cookies onto wire racks and let the cookies cool completely. These cookies are delicate, so wait until they are completely cool and peel the paper away from the cookies very gently. Store in an airtight container for up to 3 days.