Preheat oven to 325 degrees. Line 2 baking sheets with parchment paper. Thoroughly stir together confectioners’ sugar, cocoa, flour and salt in a large bowl. Beat in one egg white at a time with electric mixer on low speed. Add vanilla extract and beat for 1½ min. on high speed, scraping down sides of bowl several times. Fold in pecans, chocolate and cacao nibs until evenly incorporated. Let cookie dough rest for 5 min. Spoon the dough by heaping tablespoons on to a prepared baking sheet, about 2 inches apart. Bake cookies until dry and glossy on surface but soft in the center when pressed, 14-17 min. Let cookies cool on pan for 5 to 10 min. Carefully transfer to a wire rack to cool completely.