3/4 cup unsweetened Dutch-process cocoa powder, such as Droste
2 1/2 cups powdered sugar
1/8 tsp salt
1 Tbs pure vanilla extract
4 large egg whites, at room temperature
1 3/4 cups walnuts, coarsely chopped
1/4 cup cocoa nibs, preferably Scharffen-Berger
Preheat the oven to 350 degrees Fahrenheit, and set a rack to the middle position. Line a large baking sheet with parchment paper.
In a medium mixing bowl, whisk together the cocoa powder, sugar, and salt, taking care that there are no lumps.
Combine the vanilla and egg whites in a measuring cup or mug, and slowly add them to the cocoa mixture, beating at low speed. When you have added all the liquid, continue to beat at medium speed for about 2 minutes, until the batter is glossy and smooth. It will be fairly thin but quite viscous. Stir in the walnuts and cocoa nibs.
Using a medium (roughly 2 Tbs) ice cream scoop, place 5 mounds of batter about 3 inches apart on the baking sheet. The batter will spread. Slide the pan into the oven, reduce the heat to 325 degrees, and bake the cookies until small, thin cracks appear on their surface, about 14-17 minutes. Remove them from the oven, and cool them completely on a rack. When they are cool, peel them gently from the parchment paper. Repeat the process with the remaining batter, increasing the oven temperature to 350 degrees before each round. It should take three rounds total to use all the batter.