Preheat oven to 350 °. Grease 12 muffin cups, or line with paper muffin liners.
Combine flour, sugar, baking soda, cinnamon, and salt into a large bowl. Mix egg, oil, milk, lemon juice and vanilla extract in a 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 a toothpick inserted into the center of a muffin comes out clean 20-25 minutes.