1Place racks in lower and upper thirds of oven; preheat to 350°F. Whisk powdered sugar, cocoa powder, and salt in a large bowl, then whisk in egg whites and egg; fold in chocolate and cacao nibs. Spoon batter by the tablespoonful onto 2 parchment-lined baking sheets, spacing 2" apart.
2Bake, rotating sheets once, until cookies are puffed, cracked, and set just around the edges, 14–16 minutes.
3Transfer baking sheets to wire racks and let cookies cool on pan (they'll firm up).
4DO AHEAD: Cookies can be baked 3 days ahead. Store airtight at room temperature.