Preheat oven to 350°F. Grease and flour two 8x4-inch loaf pans or 4 smaller pans.
Whisk the sugar, eggs, oil, and vanilla together in a large bowl.
2 cups (400g) granulated sugar, 3 large eggs, 1 cup vegetable oil, 1 teaspoon pure vanilla extract
Stir in the grated zucchini, lemon juice, lemon zest, and poppy seeds.
2 1/2 cups grated zucchini, 3 Tablespoons freshly squeezed lemon juice, 2 Tablespoons lemon zest, 2 Tablespoons poppy seeds
Stir in the flour, cornstarch, baking powder, baking soda, and salt, just until combined. Divide evenly between the two loaf pans.
3 1/3 cups (416g) all-purpose flour, 2 Tablespoons cornstarch, 1 1/2 teaspoons baking powder, 1 teaspoon baking soda, 1 teaspoon table salt
Bake for 50-60 minutes until a toothpick inserted into the center of the loaves comes out clean.
Cool for 20 minutes, then turn out onto a wire rack and cool completely.
Whisk the powdered sugar with enough lemon juice to make a thin icing. Spread over the top of the loaves, then let set before slicing and serving.
2 cups powdered sugar, 1-2 Tablespoons lemon juice, extra lemon zest for garnish