Preheat oven 350 degrees. Prepare a jelly roll pan by spraying a half sheet pan with cooking spray. Set aside.
Blend together sugar, oil, and eggs.
In a separate bowl combine flour, salt, baking soda, cocoa, and cinnamon and baking powder.
With the mixer on medium, add flour mixture to the sugar and egg mixture. Then slowly add milk and zucchini until well combined.
Pour into prepared pan. Bake for 18-20 minutes.
In a microwave safe medium sized mixing bowl add the butter, cover, and heat for 1 minute in the microwave.
Remove from microwave. Stir in vanilla and buttermilk, place back in the microwave and heat for 30 seconds, stir, then heat for another 30 seconds, stir, heat for 30 seconds and stir and then heat for another 30 seconds.
Remove from microwave and stir in cocoa and powder sugar, until well combined. Place back in the microwave and heat for 30 seconds, stir. Heat for another 30 seconds and stir, repeat 1 more time if needed until frosting is smooth and there are no clumps.
Pour over brownies, and spread evenly.
Let frosting cool and harden. Cut and serve. Enjoy!