Add the first 11 ingredients and 1/2 cup of the chocolate pieces to a 1-1/2- or 2-pound bread machine according to the manufacturer's directions. Select the dough cycle. When cycle is complete, remove dough. Punch down. Cover and let rest for 10 minutes.
Divide dough in half. On lightly floured surface, roll each portion into a 10x7-inch rectangle. Sprinkle each with 2 tablespoons chocolate pieces and 2 tablespoons walnuts; press lightly. Starting from a short side, roll into a spiral; seal seam and ends. Place each loaf, seam down, in a greased 8x4x2-inch loaf pan. Cover and let rise in a warm place for 30 to 45 minutes or until nearly double.
Bake in a 350 degree F oven about 30 minutes or until bread sounds hollow when lightly tapped. If necessary, loosely cover with foil the last 10 minutes to prevent overbrowning. Remove from pans; cool slightly on wire racks. In a small saucepan combine remaining chocolate pieces and the shortening; heat over low heat until melted. Drizzle over loaves.