Heat oven to 350˚F. Line 2 large baking sheets with parchment paper. Spray the parchment paper lightly with cooking spray.
Mix sugar, cocoa, and salt in a bowl. Stir in chocolate chips. Add egg whites and vanilla; beat with a fork or electric mixer on medium until batter is just moistened.
(Do not overbeat batter or it will stiffen.)
Drop batter by teaspoonful onto baking sheets in evenly spaced mounds. Bake cookies until tops are lightly cracked and glossy, about 15 minutes. Cool briefly, then carefully remove from parchment paper with a spatula.
Leave the cookies on the paper lined sheet pans until they are cool, then remove to a storage container.
Store in an airtight container at room temperature for up to 1 week. Makes about 4 dozen cookies.