Preheat oven to 325 degrees. Line 2 baking sheets with parchment paper. Thoroughly stir together powdered sugar, cocoa, flour, and salt in a large bowl. Beat in one egg white at a time with an electric mixer on low speed. Add vanilla extract and beat for 1 1/2 min. on high speed, scraping down the sides of the bowl several times. Fold in pecans and chocolate chips until evenly incorporated. Let cookie dough rest for 5 min. Spoon the dough by heaping tablespoons onto a prepared baking sheet, about 2 inches apart. Bake cookies until dry and glossy on the surface but soft in the center when pressed, about 14 to 17 min. Let cookies cool on pan for 5 to 10 min. Carefully transfer to a wire rack to cool completely.