Preheat oven 350 degrees. Prepare a jelly roll pan by spraying a baking sheet 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, mix in the flour mixture. Then slowly adding milk and zucchini. Mix until well combined. Pour into prepared pan.
Mix well and pour into prepared pan. Bake for 18-20 minutes.
In a medium glass mixing bowl, add the butter, cover with a plate, and heat butter for 1 minute in the microwave. Remove from microwave and remove the lid. 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.