In a mixing bowl, combine flour, sugar, cinnamon, salt, baking powder and soda. Combine oil and eggs; add to dry ingredients and mix well. Add zucchini; stir until thoroughly combined. Fold in walnuts if desired. Pour into Pam sprayed 9 x 13 pan. Bake at 350 degrees for 35 to 40 minutes or until a toothpick inserted near the center comes out clean. Cool.
In a small mixing bowl, beat cream cheese, butter, milk and vanilla until smooth. Add powdered sugar and mix well. Frost cake. Sprinkle with nuts if desired. Store in refrigerator.