Step 1

Place 3 cups powdered sugar, 2/3 cup unsweetened cocoa powder, 2 teaspoons cornstarch, and 1/2 teaspoon kosher salt in a large bowl and whisk to combine. Add 2 large egg whites and 1 tablespoon vanilla extract. Stir with a rubber spatula until no lumps of cocoa or powdered sugar remain, and you have a loose, shiny dough similar to a very thick brownie batter. At first the mixture will seem too dry to combine, but will eventually come together. If after 3 minutes of mixing the dough is still too dry, stir in the remaining egg white.

Step 2

Stir in 3/4 cup semi-sweet chocolate chips. Cover the bowl with plastic wrap. Refrigerate for 1 hour.

Step 3

Arrange 2 racks to divide the oven into thirds and heat the oven to 350°F. Line 2 baking sheets with parchment paper and coat with cooking spray.

Step 4

Divide the dough into 12 portions (about 2 heaping tablespoons each). Using your hands, roll each portion into a ball. Arrange them evenly on the prepared baking sheets, 6 per sheet. Sprinkle with flaky salt if desired.

Step 5

Bake for 7 minutes. Rotate the baking sheets between racks and from front to back. Bake until the tops of the cookies are cracked, shiny, and no longer look wet, 5 to 7 minutes more. Let cool completely on the baking sheets, at least 1 hour. (The cookies will stick to the parchment paper when warm but will release when cooled.)