Heat the oven to 350°F (175°C). Line baking sheets with parchment paper or silicone baking mats, and do not grease the pans.

Whisk together the confectioners’ sugar, cocoa powder, and fine salt in a mixing bowl, breaking up any lumps.

Add the egg whites and vanilla extract. Stir until a thick, glossy, sticky batter forms; do not whip the egg whites separately.

Fold in the semisweet chocolate chips until they are evenly distributed throughout the batter.

Drop the batter by rounded tablespoons, using about 2 tablespoons per cookie, onto the prepared pans. Leave at least 2 inches between portions because the cookies spread substantially.

If the batter seems unusually thin, let it stand for 10–30 minutes, especially if the kitchen is warm. Scoop smaller portions if needed and keep plenty of room between cookies.

Bake for 8–10 minutes, until the tops are shiny and cracked and the edges look set. The centers should still look slightly underdone.

Cool the cookies on the baking sheet for several minutes before transferring them to a rack. They firm as they cool while remaining soft and fudgy in the center.