Place an oven rack on the center shelf and preheat the oven to 350 degrees F. Grease a 9x13-inch baking dish with baking spray.

In a medium-sized bowl, whisk together the flour, baking soda, baking powder, salt, and cinnamon.

In a large bowl, whisk together the eggs, oil, butter, sugar, brown sugar, and vanilla, then stir in the zucchini.

Fold the dry mixture into the wet mixture (do not overmix).

Pour the batter into the prepared baking dish, then bake for 30-35 minutes, until a toothpick comes out clean. Cool completely before frosting.

To prepare the frosting, beat together the butter and cream cheese in a bowl using an electric hand mixer or a stand mixer fitted with the paddle attachment. Add the cinnamon, nutmeg, salt, and vanilla, mixing until smooth. On low speed, gradually add the confectioners’ sugar until smooth, scraping down the sides of the bowl periodically with a spatula.

Frost the cake, top evenly with the walnuts, and serve.