Method
1. Place racks in lower and upper thirds of oven; preheat to 350°F/ 175°C
2. Add powdered sugar, cocoa powder and salt to a clean large mixing bowl
3. Whisk briefly until combined
4. Add egg white and egg to the batter
5. Whisk until combined
6. Add bittersweet chocolate to the batter
7. Fold gently
8. Line 2 clean baking sheets with parchment paper
9. Scoop batter onto 2 baking sheets
10. Bake, rotating sheets once, until cookies are puffed, cracked, and set just around the edges, 14–16 minutes
11. Transfer baking sheets to wire racks and let cookies cool on pan