Preheat oven to 325 degrees. Whisk together confectioners' sugar, cocoa, and salt. Stir in chocolate and nuts. Add egg whites; stir until just incorporated.
Drop dough by 1/4 cupfuls, 3 inches apart, onto 2 parchment-lined rimmed baking sheets. Bake until cookie tops are dry and crackled, 22 to 25 minutes, rotating sheets halfway through. Transfer baking sheets to wire racks; let cookies cool completely.
Cookies can stored in an airtight container 3 days.