Preheat the oven: Set the oven to 350°F (175°C). Line an 8×8-inch baking dish with parchment paper, leaving a bit of overhang for easy removal later. Lightly grease the parchment if desired to prevent sticking.
Mix the wet ingredients: In a large mixing bowl, whisk together the melted butter, brown sugar, and granulated sugar until the mixture is smooth and glossy. Add the egg and vanilla extract, whisking until fully incorporated and slightly thickened. This step ensures the base of the batter is rich and cohesive.
Combine dry ingredients: In a separate bowl, whisk the flour, baking soda, baking powder, cinnamon, and salt. Mixing these dry ingredients separately prevents uneven distribution of leavening agents and ensures the bars bake evenly.
Blend wet and dry ingredients: Add the dry mixture to the wet ingredients. Stir gently with a spatula or wooden spoon until just combined. Avoid overmixing — a few streaks of flour are fine at this stage — to keep the bars tender and soft.
Add zucchini and chocolate chips: Fold in the well-drained zucchini and chocolate chips until evenly distributed. Make sure to squeeze out excess water from the zucchini beforehand to avoid excess moisture, which can make the bars gummy instead of chewy.
Spread the batter: Pour the thick batter into the prepared baking dish and spread evenly with a spatula, smoothing the top. Press lightly to level out any uneven spots so the bars bake uniformly.
Bake: Place in the preheated oven and bake for 25–30 minutes, or until the top is golden brown and a toothpick inserted into the center comes out with a few moist crumbs. The edges should look set and slightly crisp while the center remains soft.
Cool and slice: Allow the bars to cool completely in the pan on a wire rack before slicing. Cooling helps the structure firm up and makes cutting easier. Lift the bars from the pan using the parchment overhang, then slice into even squares or rectangles.
