Preheat the oven to 350°F (175°C).

Grease a 9×5-inch loaf pan and line with parchment paper, leaving some overhang for easier lifting.

Whisk the eggs, vegetable oil, melted butter, granulated sugar, brown sugar, and vanilla extract in a large bowl until smooth and glossy, about 30 seconds. Then stir in the grated zucchini until evenly distributed.

In a separate bowl, combine the all-purpose flour, baking soda, baking powder, cinnamon, and salt. Mix until the dry ingredients look uniform in color.

Gradually mix the dry ingredients into the wet ingredients until just combined, stopping as soon as no dry streaks remain. The batter should look thick and slightly lumpy.

Pour the batter into the prepared loaf pan and smooth the top. Mix the cinnamon and granulated sugar for the topping, then sprinkle evenly over the batter so it forms an even speckled layer.

Bake at 350°F (175°C) for 50–60 minutes, until a toothpick inserted in the center comes out clean. If the top browns too quickly, loosely tent with foil for the remaining time.

Cool in the pan for 15 minutes so the loaf sets. Transfer to a wire rack and cool completely before slicing to keep the crumb tender.