Preheat oven to 350F.
Toast your walnuts or pecans, cool them and roughly chop them. In a large bowl, stir together the icing sugar, cocoa and salt. Add the nuts, then stir in the egg whites and vanilla. Stir until well combined.
Line a baking sheet with parchment and drop the batter in large mounds (about 2 Tbsp. each - I used a heaped soup spoon) spacing them well away from each other - no more than 6 cookies per sheet. Bake for 13-15 minutes - they will spread, puff, crack on top, get glossy and then turn matte. Slide the cookies on the parchment off the sheet onto a cooling rack and let them cool.