Preheat your oven to 350°F (175°C). Lightly grease a 9x13-inch baking dish and set aside.
In a medium bowl, whisk together the flour, cinnamon, baking soda, salt, and baking powder until evenly combined.
In a large separate bowl, stir together the grated zucchini, sugar, vegetable oil, lightly beaten eggs, and vanilla extract until well mixed.
Pour the dry ingredients into the wet ingredients and stir until just combined — stop as soon as no dry flour remains. Do not overmix.
Fold in the chopped walnuts, then pour the batter into the prepared baking dish and smooth the top evenly with a spatula.
Bake for 45 minutes, or until a toothpick inserted into the center comes out clean. Allow to cool in the pan before slicing and serving.