Set your oven to 350°F (175°C). Lightly grease a 9×13-inch rectangular baking dish and line it with parchment paper, allowing extra overhang for easy lifting.
In a separate large bowl, combine the oil, sugar, eggs, and vanilla. Whisk thoroughly until the mixture is cohesive and smooth.
In a different bowl, sift together the flour, cocoa powder, baking soda, and salt to ensure even distribution of the dry ingredients.
Incorporate the dry ingredients into the wet mixture in increments, alternating with the milk. Stir only until the batter is just combined—avoid overmixing.
Fold the grated zucchini gently into the batter. Once evenly distributed, fold in the chocolate chips or chunks. The mixture will appear thick initially but will loosen as the zucchini releases its moisture.
Pour the batter into the prepared baking pan and level the surface with a spatula. Bake for 30 to 35 minutes, or until a toothpick inserted in the center emerges with a few moist crumbs clinging to it.
Let the brownies rest in the pan for no less than 20 minutes before removing and slicing into squares. Serve as desired.