Preheat your oven to 350°F (175°C) and prepare two 8-inch round cake pans by greasing and lightly flouring them.

In a bowl, whisk together flour, baking powder, baking soda, salt, cinnamon, and nutmeg until well combined.

In a large bowl, beat together vegetable oil, granulated sugar, and brown sugar until smooth. Add in eggs one at a time, beating well after each. Mix in vanilla extract.

Gradually add dry ingredients to wet, stirring gently to combine. Fold in grated carrots, zucchini, and walnuts if using.

Evenly divide batter between prepared pans and smooth the tops.

Bake for 25-30 minutes, or until a toothpick inserted comes out clean. Let cool in pans for 10 minutes before transferring to a wire rack.

In a bowl, beat cream cheese and butter until creamy. Gradually add powdered sugar and vanilla, mixing until fluffy.

Once cakes are cool, frost the top of one layer, place the second layer on top, and frost the top and sides evenly with remaining frosting.

Garnish with chopped walnuts if desired and serve at room temperature.