1. Preheat your oven to 350°F. Grease a 9×13-inch baking pan.

2. In a large bowl, whisk together the all-purpose flour, unsweetened cocoa powder, baking soda, and salt.

3. In a separate bowl, combine the vegetable oil, granulated sugar, and vanilla extract. Stir in the shredded zucchini until well incorporated. Gradually add the dry ingredients to the wet ingredients, mixing until just combined. If the batter seems too dry, add water one tablespoon at a time until it reaches a thick, spreadable consistency. Finally, fold in the chopped walnuts (if using).

4. Spread the brownie batter evenly into the prepared baking pan. Bake for 25–30 minutes, or until a toothpick inserted in the center comes out with moist crumbs. Allow the brownies to cool completely before frosting.

5. In a medium bowl, whisk together the unsweetened cocoa powder and melted unsalted butter. Gradually stir in the powdered sugar, milk, vanilla extract, and a pinch of salt until the frosting is smooth and creamy.

6. Spread the chocolate frosting evenly over the cooled brownies. Cut into squares and serve.