1/2 cup plus 3 tablespoons unsweetened Dutch-process cocoa powder
1/4 teaspoon salt
4 large egg whites, at room temperature
1 tablespoon pure vanilla extract
Preheat the oven to 350°. Position 2 racks in the upper and lower thirds of the oven. Line 2 large rimmed baking sheets with parchment paper.
Spread the walnut halves on a large rimmed baking sheet and toast in the oven for about 9 minutes, until they are golden and fragrant. Let cool slightly, then transfer the walnut halves to a work surface and finely chop them.
In a large bowl, whisk the confectioners' sugar with the cocoa powder and salt. Whisk in the chopped walnuts. Add the egg whites and vanilla extract and beat just until the batter is moistened (be careful not to overbeat or it will stiffen). Spoon the batter onto the baking sheets in 12 evenly spaced mounds.
Bake the cookies for about 20 minutes, until the tops of the cookies are glossy and lightly cracked and feel firm to the touch; shift the pans from front to back and top to bottom halfway through.
Slide the parchment paper (with the cookies) onto 2 wire racks to cool completely before serving.
The cookies can be stored in an airtight container for up to 3 days.