Preheat your oven to 350°F (175°C). Grease two 8-inch round cake pans with butter or cooking spray, then dust with flour.

In a large mixing bowl, whisk together the dry ingredients: flour, baking powder, baking soda, salt, cinnamon, and nutmeg.

In a separate bowl, mix vegetable oil, granulated sugar, and brown sugar. Add eggs one at a time, then mix in vanilla extract.

Gradually fold the dry mixture into the wet mixture until just combined, being careful not to overmix.

Gently fold in grated carrots and zucchini, and walnuts if using.

Pour the batter into the prepared pans and bake for 25 to 30 minutes. Check for doneness with a toothpick.

Allow the cakes to cool in the pans for 10 minutes before transferring to a wire rack to cool completely.

Prepare the frosting by beating cream cheese and unsalted butter until smooth, then gradually mix in powdered sugar and vanilla extract.

Once the cakes are cool, frost the top of one layer, stack the second layer on top, and frost the top and sides of the cake.

Garnish with additional walnuts if desired and serve.