Preheat the oven to 400°. Toast the walnuts for about 8 minutes, or until golden brown. Let the nuts cool, then coarsely chop them. Lower the oven temperature to 325°.
In a large bowl, sift together the flour, baking powder and salt. In a large bowl, cream the butter with the sugar. Beat in the eggs, 1 at a time, blending thoroughly after each addition. Add the vanilla. Gradually stir in the flour mixture, then fold in the nuts and the chocolate until just blended. Divide the dough into 4 pieces and shape each piece into a 12-inch log. Set the logs on 2 lightly buttered baking sheets and refrigerate until chilled, at least 30 minutes.
Bake the dough logs for about 20 minutes, or until they are firm and starting to brown. Transfer the logs to a rack and let cool slightly. Lower the oven temperature to 250°. Using a serrated knife, cut the logs crosswise diagonally, 1/2 inch thick. Set the cookies on their side on the baking sheets and bake for about 15 minutes, or until golden brown. Let cool before serving.
The chocolate-packed biscotti can be stored in an airtight container for up to 3 days.