Mix together sugar, cocoa, cornstarch and salt. Gradually add egg whites, stirring with a spoon until mixture forms a dough. (Mixture should be thick enough to form into balls; if not add more powdered sugar and cocoa). Add nuts; mix well.
Form dough into 15 balls. Place on a parchment-lined cookie sheet. Bake 16 to 19 minutes, until glossy and crackled. Cool completely.