About 2 hours before serving or early in day: In 10-inch skillet over medium heat, toast walnuts, shaking skillet frequently, until golden; remove skillet from heat. Cool walnuts.
Meanwhile, into large bowl, measure flour, sugar, cocoa, expresso-coffee powder, baking soda, baking powder, and salt.
Chop walnuts. With spoon, stir walnuts, egg substitute, and vanilla extract into flour mixture just until blended. With floured hands, knead mixture until it holds together (mixture may appear dry or crumbly).
Preheat oven to 325 F. Spray large cookie sheet with nonstick cooking spray. On lightly floured surface, divide dough in half. With lightly floured hands, roll each half of dough into an 8" by 2 1/2" loaf. Place loaves, about 3 inches apart, on cookie sheet. Bake 35 to 40 minutes until toothpick inserted in center of loaf comes out clean.
Remove cookie sheet from oven. Turn oven control to 300 F. Cool loaves 10 minutes for easier slicing.
With serrated knife, cut loaves crosswise into 1/4 inch thick diagonal slices. Arrange slices, cut-side down, on same cookie sheet, making sure they do not touch. Return to oven and bake 10 to 15 minutes longer (cookies will seem cakelike in center but will become characteristically crisp and dry when cooled). Remove cookies to wire racks to cool. Store cookies in tightly covered container. Biscotti are traditionally dipped in hot coffee for dessert.Servings: 3 1/2 dozen cookies