Preheat oven to 300 degrees F. Cover baking sheet with parchment paper; draw two circles, each 7 to 7-1/2 inches.
Measure flour, cocoa, sugar and salt into the food processor or into the large bowl of electric mixer. Mix well. Cut the butter into thin slices and add to the flour mixture. Mix or process until mixture resembles moist sand. Add the chocolate and nuts and continue mixing until a dough forms. Shape dough into a smooth ball. Cut the dough into quarters. Shape each quarter into a smooth ball. Place dough ball onto the center of each circle drawn on the parchment paper. Flatten the round to within 1/2 inch of the inside edge of the round. With a fork, make impressions on the top of each round and use the fork to round off the edges. Sprinkle each round with 1 teaspoon sugar. With a straight knife, cut each round into eight wedges; leave wedges in place.
Bake for 1 hour or until the shortbread is a light golden color. With a straight-edged knife, cut the wedges apart while still warm.