Heat oven to 350. Finely chop walnuts and roast 9 minutes on baking sheets lined with parchment paper. Let cool. Mix sugar, salt, and cocoa. Stir in walnuts. Add egg whites and vanilla. Beat with fork or mixer on medium until just moistened. Do not overbeat. Drop by teaspoonful onto baking sheets. Bake until tops are cracked and glossy, about 15 minutes. Store in airtight container for up to one week.