Adjust oven rack to middle position and heat oven to 350 degrees. Using ruler and pencil, draw a 4 by 12 inch rectangle on a piece of parchment. Place the pencil side down on a baking sheet and spray the unmarked side with vegetable oil.
Pulse flour, cocoa powder, baking soda, and salt in food processor to combine, about 15 seconds. Transfer the flour mixture to a bowl. Process 2 eggs in now empty food processor until lightened in color and almost doubles in volume, about 3 minutes. With processor running, slowly add sugar until thoroughly combined, about 15 seconds. Add melted butter and process until combined, about 10 seconds. Transfer egg mixture to medium bowl. Stir in flour mixture to form a stiff dough. Stir in walnuts and chocolate chips.
Using floured hands, form the dough into a 12 by 4 inch rectangle using the penciled lines as a guide. Spray the loaf lightly with oil spray. Using rubber spatula lightly coated with oil spray, smooth top and sides of rectangle. Using a sharp knife coated with oil spray, cut the loaf in half making it into two - 2 by 12 inch loaves. Separate loaves leaving 2 inch space between them. Sprinkle with confectioners' sugar. Bake loaves for 35 minutes, or until slightly firm to the touch.
Let loaves cool on baking sheet for 25 minutes. Transfer loaves to cutting board. gently spray water on top of loaf and serrated knife. Slice each loaf on a slight bias into 3/4-inch-thick slices. Lay slices, cut side down, about 1/4 inch apart on wire rack set in rimmed baking sheet. Bake until crisp and golden brown on both sides, about 35 minutes. Let cool completely before serving. Biscotti can be stored in airtight container for up to 1 month.