Preheat oven to 400 degrees F. Lay pecans on a rimmed cookie sheet and toast for 6-8 minutes or until they start to release their oils and become shiny. Let the pecans cool, then coarsely chop.
Have ready two large baking sheets lined with parchment.
In a medium bowl, stir or sift (I just stir) together the powdered sugar, cocoa powder and salt.
In a large metal or glass mixing bowl, beat the egg whites with an electric mixer (I use a handheld) until soft peaks start to form.
Turn off mixer and using a heavy duty scraper, stir in powdered sugar/cocoa mixture, scraping sides of bowl. With the electric mixer, beat on high for minute or until smooth and shiny (mixture will not be stiff, but more like a thick glue). Beat in vanilla. Fold in the pecans. The mixture may still be runny at this point, so you may need to work quickly.
Drop the nut mixture onto the baking sheets (6 to a sheet) by rounded tablespoonfuls spacing about 2 inches apart. Bake the cookies or 5 minutes at 400,
Reduce heat to 325 and continue to bake the cookies for another 8 to 10 minutes or until they appear dry and set.
Allow the cookies to cool completely on the baking sheet, then peel away from the parchment.
I like to make these with good quality natural cocoa powder, but Cocoa Rouge, a Dutch chocolate powder, also works well.