Preheat your oven to 350°F (175°C). Grease and flour the 9x13 inch baking pan.
In a large mixing bowl, combine the flour, sugar, baking soda, baking powder, salt, cinnamon, and nutmeg. Mix well.
In a separate bowl, whisk together the oil, eggs, and vanilla extract until combined.
Gradually add the wet ingredients to the dry ingredients, stirring until just combined.
Fold in the grated zucchini and, if desired, the chopped nuts.
Pour the batter into the prepared baking pan and spread it evenly.
Bake for 30-35 minutes or until a toothpick inserted in the center comes out clean. Allow the cake to cool completely in the pan on a wire rack.
While the cake is cooling, prepare the cream cheese frosting. In a mixing bowl, beat the softened cream cheese and butter until smooth and creamy.
Gradually add the powdered sugar and mix until combined. Add the vanilla extract and enough milk to reach your desired consistency.
Once the cake is cooled, spread the cream cheese frosting evenly over the top.
Cut into squares and serve.