Preheat oven to 180°C. Put toasted, roughly chopped walnuts, powdered sugar, cocoa powder and salt in a bowl. Give them a little whisk so all the ingredients are combined and then start adding the egg whites and vanilla. Mix all the ingredients with a mixer on medium speed until the mixture starts to get thicker, about 3 minutes. Don't over-mix it or the egg whites will thicken too much.
Line a baking sheet with parchment paper and scoop about 10 cookies that are 8 cm in diameter, at least 3 cm apart so that they don't stick together when they spread. Don't worry if they look very flat, they will puff up. When you put them in the oven, lower the oven temperature to 160°C.
Bake them for about 15 min or until small, thin cracks appear on the surface of the cookies. I know I say this every time but it is so important not to over-bake or your cookies will be unpleasantly chewy. When they are baked pull the parchment paper off the baking sheet and let them cool.