Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking pan with butter and dust lightly with flour.

In a medium bowl, whisk together flour, baking soda, baking powder, cinnamon, nutmeg, and salt.

In a large bowl, combine vegetable oil, granulated sugar, brown sugar, eggs, and vanilla extract. Mix until smooth.

Gradually add the dry mixture to the wet ingredients, stirring gently until just combined.

Fold in grated zucchini and optional chopped walnuts until evenly distributed.

Pour the batter into the prepared pan and bake for 40-45 minutes or until a toothpick comes out clean.

Let the cake cool in the pan on a wire rack for 15-20 minutes, then transfer to cool completely.

For the frosting, beat together cream cheese and butter until smooth. Gradually add powdered sugar and vanilla until well combined.

Spread the cream cheese frosting evenly over the cooled cake.