Preheat oven to 325 degrees. Line 2 baking sheets with parchment paper. Thoroughly stir together confectioner’s 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 minutes 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 minutes. Spoon the dough by heaping tablespoon 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 minutes. Let cookies cool on pan for 5-10 minutes. Carefully transfer to a wire rack to cool completely.