Spread chopped walnuts on another baking sheet and toast in the oven, about 8 minutes, until they are golden and fragrant. Let cool slightly.
Turn oven down to 325 degrees.
In a large bowl, whisk together powdered sugar with the cocoa powder and salt, then whisk in chopped walnuts. Whisk in egg whites and vanilla extract and beat just until the batter is moistened. Beating too much will make the cookies stiff.
Spoon batter in mounds onto the prepared baking sheets and bake for 14 minutes, until the tops are glossy and lightly cracked. Rotate pans halfway through to ensure even baking.
Slide cookies and parchment paper onto two wire racks. Sprinkle cookies with additional large-flake sea salt if desired. Let cookies cool completely before serving.