Preheat oven to 350 degrees F. If you plan on chilling the dough you can skip this step.
Combine flour, baking soda and salt in medium bowl; set aside.
With a heavy duty scraper or with an electric mixer, beat together the softened butter, sugars and the egg. Do not overbeat or attempt to make the mixture fluffy, just beat until mixed. Beat in the cocoa powder.
Stir in flour mixture just until blended, then stir in chocolate and nuts.
Using a generous medium size cookie scoop, scoop mounds of dough onto dinner plates lined with plastic wrap. Cover and chill dough balls or bake right away. Cookies made with chilled dough should have a more rounded, uniform shape.
Bake on parchment lined baking trays, spacing cookies 2 ½ inches apart, for about 14 minutes (check at about 10) or until they appear set. Slide parchment off tray and let cool on the parchment on the counter.