Preheat the oven to 350°F (175°C). Line an 8 x 8-inch (20 x 20 cm) square metal baking pan with parchment paper, leaving an overhang on two opposite sides. This will make it easier to lift the brownies out after cooling. Make sure the parchment lies flat against the bottom and corners so the batter spreads evenly.

Wash the zucchini and trim the ends: Leave the skin on, then finely shred it using a box grater. Measure 1 cup (125 g) of lightly packed shredded zucchini. Do not squeeze, drain, or pat the zucchini dry. Its natural moisture is an important part of the batter. The shreds should be fine enough to blend into the brownies rather than remain as large, noticeable pieces.

In a large mixing bowl, combine: - 1/2 cup (113 g) melted unsalted butter - 3/4 cup (150 g) packed light brown sugar - 1/4 cup (50 g) granulated sugar Whisk until the mixture looks thick, glossy, and evenly combined. It may still feel slightly grainy because the sugar will not dissolve completely at this stage.

Add 2 large eggs (about 100 g without shells) and 2 teaspoons (10 ml) vanilla extract to the butter mixture. Whisk until the mixture becomes smoother, slightly thicker, and glossy. The eggs should be fully incorporated, with no visible streaks. Avoid prolonged vigorous whisking, which can incorporate excess air and encourage a lighter, more cake-like texture.

Add the prepared 1 cup (125 g) shredded zucchini. Fold it into the wet mixture with a silicone spatula until the strands are distributed evenly. The mixture may look slightly loose or uneven because the zucchini releases moisture. This is expected.

In a medium mixing bowl, whisk together: - 3/4 cup (90 g) all-purpose flour - 1/2 cup (43 g) unsweetened cocoa powder - 1/4 teaspoon (about 1.5 g) fine salt Break up any cocoa lumps and mix until the dry ingredients are uniform in color. This helps prevent dry flour pockets and concentrated patches of cocoa in the finished brownies.

Add the dry ingredients to the zucchini mixture: Using a silicone spatula, fold gently until no dry flour or cocoa streaks remain. The batter should look thick, dark, and chocolatey, with small zucchini strands distributed throughout. It should be spreadable rather than freely pourable. If the mixture initially looks unusually stiff, continue folding gently to distribute the zucchini moisture before deciding that anything is wrong. Avoid adding extra liquid. Stop mixing as soon as the batter is combined. Excessive mixing can develop gluten and make the brownies tougher.

Add 1/2 cup (87 g) semisweet chocolate chips to the batter. Fold just until the chocolate chips are distributed throughout. Reserve the remaining 1/4 cup (43 g) for the surface. The batter should remain thick enough to hold the chocolate chips without them immediately sinking.

Spoon the batter into the prepared 8 x 8-inch (20 x 20 cm) baking pan. Spread it into an even layer, working gently into the corners. Sprinkle the reserved 1/4 cup (43 g) chocolate chips evenly over the surface and press them lightly into the batter. An even surface helps the brownies bake consistently from the edges toward the center.

Place the pan on the center oven rack: Bake at 350°F (175°C) for approximately 30 minutes. Look for these signs of doneness: - The edges appear set and slightly firm. - The surface looks mostly matte rather than wet. - The center no longer moves like liquid batter when the pan is gently nudged. - A toothpick inserted into the center comes out with moist, sticky crumbs rather than raw batter. Avoid inserting the toothpick directly into a melted chocolate chip, which can make the brownies appear underbaked. A completely clean toothpick is not the goal for fudgy brownies. If the center is still visibly liquid, continue baking briefly and check again. Actual baking time can vary with the pan material, oven, and moisture content of the zucchini.

Remove the pan from the oven and place it on a wire rack. Allow the brownies to cool in the pan for 1 hour 30 minutes before slicing. During cooling, the center becomes firmer and the chocolate chips begin to set. Cutting too early can cause the brownies to break apart or appear wetter than they will after cooling.

Once cooled, lift the brownies out using the parchment overhang. Transfer them to a cutting board and cut into 16 squares. Use a sharp knife and wipe the blade between cuts for cleaner edges. The finished brownies should have a rich chocolate color, a soft, dense center, and a moist crumb that holds together when lifted.