Preheat oven to 300 degrees, with racks in upper and lower thirds. Line two large baking sheets with parchment paper; set aside. In a small bowl, whisk together sugar and cornstarch; set aside.
In a large bowl, using an electric mixer, beat egg whites and salt on medium speed until frothy. Beating constantly, add sugar mixture 1 tablespoon at a time; beat until stiff, glossy peaks form, 6 to 8 minutes total (scrape down the bowl halfway through). Add espresso powder and cocoa; beat until well blended. With a rubber spatula, fold in chocolate chips.
Drop batter by level tablespoons onto prepared baking sheets, about 1 inch apart. Bake until crisp, about 40 minutes, rotating sheets halfway through. Cool completely on sheets, about 20 minutes.