Preheat the oven to 350 degrees. Prepare a 9" x 13" baking pan with cooking spray.
Grate the zucchini and set aside on a layer of paper towels. Press down with more paper towels on top to remove any water.
In a mixing bowl, cream together butter, oil, eggs, sugar and vanilla, for about 3 minutes.
Add in buttermilk and vanilla.
Next, mix together the flour, cocoa, soda, salt, and cardamom in a separate bowl and whisk.
Add the dry ingredients to the creamed ingredients and mix until combined.
Add grated zucchini and gently mix just. Don’t over mix, because it will make the batter runnier.
Pour the batter in the prepared baking dish.
Sprinkle brown sugar, walnuts, and chocolate chips over the top of cake.
Bake 350 degrees for 35-40 minutes. Be careful to not overbake the cake. To check for doneness stick a toothpick in the middle of the cake, it should come out clean, or with a few crumbs.