Preheat oven to 180°C/350°F/ Gas 4. Line baking tray with greaseproof paper.
In an electric mixeer bowl electric mixer fitted with the paddle attachment, mix together the cocoa powder, icing sugar, salt, and walnuts, and beat for 1 minute on low speed until well combined.
In another bowl, briefly mix together the vanilla and egg whites.
Turn up the mixer's speed to medium and gradually add the whites to the dry ingredients.
Beat for 2 minutes.Divide the mixture using a scoop onto the trays, leaving about 3" between cookies.
Place the baking trays on the middle shelf and reduce the temperature of the oven to 160°C/320°F.
Bake for 12-4 minutes or until thin cracks appear and the cookies are firm to the touch.
Leave to cool slightly and then transfer to a wire rack to cool completely.