Position 2 racks in the upper and lower thirds of oven.
Heat oven to 350°. Line 2 large baking sheets with parchment paper.
Place walnuts on a work surface and finely chop.
Transfer to a separate large baking sheet and toast until fragrant, about 9 minutes.
Let cool.
Mix sugar, cocoa and salt in a bowl. Stir in walnuts.
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 the teaspoonful onto baking sheets in evenly spaced mounds.
Bake cookies until tops are lightly cracked and glossy, about 15 minutes. Repeat with remaining batter. Store in an airtight container at room temperature for up to 1 week.
