Heat oven to 350F. 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 coarsely chop them.

 

Make the cookies

Position two racks in the upper and lower thirds of the oven and lower temperature to 325 degrees F. Line two large baking sheets with parchment paper.

In a large bowl, whisk (or combine in an electric mixer on low speed) the confectioner’ sugar with the cocoa powder and salt followed by the chopped walnuts. While whisking (or once you change the speed to medium), add the egg whites and vanilla extract and beat just until the batter is moistened (do not overbeat or it will stiffen; we’re not trying to whip these egg whites as we would for a meringue).

Spoon the batter onto the baking sheets in evenly spaced mounds — I use a just-over-1-tablespoon, or #40 scoop. If you can spare the time, letting them rest at room temperature on their trays for 30 minutes (and up to 60), I find they get a taller dome in the oven, but it’s also fine to bake them right away.

Bake for 14 to 16 minutes, until the tops are glossy and lightly cracked; shift the pans from front to back and top to bottom halfway through to ensure even baking. Slide the parchment paper (with the cookies) onto 2 wire racks. Let cookies cool completely, and store in an airtight container for up to 3 days.