Preheat oven to 325 degrees F. Grease a 9×5-inch loaf pan and line it with parchment paper so excess parchment hangs over each side.: You should notice the kitchen warming and a faint scent from the preheating oven, which helps set the batter once it goes in. Greasing and lining the pan ensures the loaf releases cleanly, avoiding tearing the crust when unmolding; the parchment overhang gives you handles for lifting. A common mistake is skipping the parchment which can lead to the top sticking and breaking when removed. If your pan is dark, reduce heat by a few degrees or tent with foil halfway through to prevent overbrowning.
Place the zucchini in a clean dish towel, gather the ends together, then twist and squeeze to drain out as much liquid as possible.: When you squeeze, you should hear a gentle squelch and feel the towel cool as liquid is expelled, which is crucial for the loaf's texture. Removing this water prevents a soggy center and concentrates the zucchini's flavor. One trap is not squeezing long enough, which leaves excess moisture and can make the crumb gummy. If the zucchini still seems watery, give it another firm twist and press over a bowl to capture the liquid.
In a large bowl, whisk together the flour, cinnamon, baking powder, baking soda, and salt to combine; set aside.: Whisking releases the aroma of the cinnamon and ensures the leaveners are dispersed, which yields an even rise and prevents pockets of baking soda. Visually, the mixture will look uniform and pale, with specks of spice distributed throughout. A common error is adding wet ingredients directly into unmixed dry pockets, causing uneven texture, so take a minute to whisk thoroughly.
In a medium bowl, whisk together the brown sugar, oil, eggs, and vanilla extract until completely combined. Fold in the drained zucchini.: As you whisk, the brown sugar dissolves into the oil and eggs, producing a glossy, cohesive mixture that smells warmly of molasses and vanilla. Folding in the zucchini introduces flecks of green and moist strands into the batter, and you should see the mixture become thicker and more textured. Beware overmixing at this stage since vigorous stirring can lead to a tougher crumb. Use gentle motions and stop when the zucchini is evenly distributed.
Add the zucchini mixture to the flour mixture and use a rubber spatula to fold together until a few streaks of flour remain. Add the chopped walnuts and gently fold to combine.: The batter will feel slightly dense yet pliable, with the flour streaks indicating you should stop mixing soon to avoid overdeveloping gluten. The aroma should shift to a nutty, spiced fragrance when the walnuts are folded in. A common misstep is overfolding which can produce a tight, less tender loaf. Aim for a uniform batter with some small streaks of flour that disappear with a few light folds.
Pour the batter into the prepared pan, smooth into an even layer, and sprinkle the granulated sugar evenly over the top.: Pouring is tactile, and you may hear a soft glug as the batter settles; smoothing the top ensures an even rise and attractive appearance. The sprinkle of granulated sugar will glint slightly and caramelize in the oven for a subtle crust. Avoid packing the batter tightly into the pan which can trap air and create dense spots, instead use gentle taps on the counter to level it.
Bake until a toothpick inserted in the center comes out with just a few moist crumbs attached, 65 to 75 minutes. Place the pan on a wire rack and allow the bread to cool in the pan for 30 minutes. Remove the bread from the pan and allow to cool completely on a wire rack before serving. Leftover bread should be wrapped tightly in plastic wrap and stored at room temperature for up to 4 days.: During baking you will smell the cinnamon deepen and the top will take on a golden brown hue, with the kitchen filling with toasty, sweet notes. The toothpick test ensures the interior is done without drying it out, and resting in the pan helps the loaf set so it does not collapse when removed. A frequent issue is removing the bread too soon, which causes a gooey center, so be patient and test multiple spots near the center.
Remove the bread from the pan and allow to cool completely on a wire rack before serving. Leftover bread should be wrapped tightly in plastic wrap and stored at room temperature for up to 4 days.: As you unmold, the crust should feel firm and slightly crisp; cooling on a wire rack prevents trapped steam from making the underside soggy. Cooling completely allows the crumb to finish setting, which makes slicing neater and keeps the texture perfect. Storing wrapped at room temperature preserves moisture, but leaving it unwrapped will dry the loaf out quickly, so seal it well to maintain freshness.