Preheat oven to 325°F.

Line 2 baking sheets with parchment paper or nonstick baking mats.

In a large bowl of an electric mixer, mix together the sugar, cocoa, flour and salt.

Beat in egg whites, one at a time, with mixer on low speed. Add vanilla and continue to beat scraping down sides several times.  Once all incorporated, fold in walnuts, chocolate and cocoa nibs.

Spoon the dough by heaping tablespoonfuls onto the prepared baking sheets, about 2 inches apart.

Bake the cookies, in batches, in the center of the oven, until dry and glossy on the surface but soft in the centers when pressed, 15 to 17 minutes. Let cool on the pan for 5 to 10 minutes. Carefully transfer the cookies, on the paper or mats, to a wire rack to cool completely.