Preheat oven to 300. Line baking sheet with parchment paper, or Silpat sheets.
In food processor with metal blade, combine chocolate and sugar and grind until the chocolate is very fine. Set aside.
In a medium bowl sift flour, cocoa powder, baking soda, and salt.
In a large mixing bowl combine eggs with vanilla mix well. Alternating, add the chocolate and flour mixture. Add nuts when half way adding the flour/ chocolate until you get a stiff dough. Taransfer dough to a floured surface and divide in half. Form each half into a log 12 inches long or so that it fits the length of your baking sheet. Transfer the logs on to the baking sheets, spacing them evenly. Pat logs so that they are slightly flattened on the top. Bake until almost firm to the touch @ 50 minutes. Let cool for 10 minutes. Transfer baked logs onto cutting board and slice logs on the diagonal about 1/2 inch in width.
Return the slices cut side down to the baking sheet. Bake for 25 minutes. Turn the slices over and bake until crisp and dry, about 25 minutes more.
Optional: Drizzle biscotti with melted dark and white chocolate.