Preheat oven to 350° (175° C). Grease 12 muffin cups, or line with paper muffin liners.
Combine flour, sugar, baking soda, cinnamon, and salt in large bowl.
Mix egg, oil, milk, lemon juice, and vanilla extra in another bowl; stir into dry ingredients until just moistened.
Fold in zucchini, chocolate chips, and walnuts. Fill prepared muffin cups 2/3 full.
Bake in preheated oven until the toothpick inserted into the center of a muffin comes out clean, about 20 – 25 minutes.
Cool and serve.