In a bowl, combine flour, sugar, baking soda, cinnamon and salt. Combine the egg, oil, milk, lemon juice and vanilla; mix well. Stir into dry ingredients just until moistened. Fold in zucchini, chocolate chips and walnuts. Fill greased or paper-lined muffin cups two-thirds full. Bake at 350 degrees F for 20-25 minutes or until muffins test done.