Preheat & Prep: Heat your oven to 350°F (175°C). Generously grease a 9x13 inch baking dish.
Whisk Dry Ingredients: In a large bowl, whisk together the flour, ½ cup cocoa powder, baking soda, and salt.
Mix Wet Ingredients: In a medium bowl, blend the vegetable oil, granulated sugar, and vanilla extract.
Combine & Rest: Pour the wet ingredients into the dry, mixing gently. Fold in the shredded zucchini. The batter will be thick. Let it rest for 5 minutes so the zucchini can release its moisture.
Adjust Batter: If the mixture is still too powdery after resting, add 3 to 5 tablespoons of water, one at a time, until a thick dough forms. Fold in the walnuts.
Bake: Spread the batter evenly into the prepared 9x13 pan. Bake for 25–30 minutes until set. Let the brownies cool completely.
Make the Frosting: In a mixing bowl, whisk together the confectioners sugar, 3 tbsp cocoa powder, melted butter, milk, vanilla, and a pinch of salt until smooth and glossy.
Frost & Serve: Spread the frosting evenly over the cooled brownies. Slice into 24 squares and enjoy!