In a separate bowl, mix flour, baking powder, baking soda, cocoa, cinnamon and salt.
Mix dry ingredients and zucchini into the wet ingredients.
Pour into a 9X13 inch cake pan and bake at 350 degrees for 30-40 minutes. Insert a toothpick at 30 minutes, if it comes out clean it is done.
Allow the cake to cool completely which can take a good 1-3 hours depending on how warm your house is. Then frost the cake and serve.
Cover leftovers and store for up to 1 week.
In the bowl of an electric mixer using paddle attachment (or using an electric hand mixer with a large bowl), beat together 8 oz cream cheese with butter on medium/high speed until creamy, scraping down the bowl as needed. Add vanilla.
Sift in 3 cups powdered sugar with 1/2 cup cocoa powder to ensure there are no lumps then add 1/4 tsp salt. Mix on low speed adding the splash of milk as it comes together. Add an additional tablespoon of milk if needed.
Notes
Chocolate zucchini cake can be baked in a bundt pan for 50-60 minutes.
This cake is great to make ahead and freeze adding the frosting before serving.