Preheat the oven to 350 degrees F (175 degrees C). Grease and flour a 9×13-inch baking pan.
Mix flour, sugar, cocoa, baking soda, baking powder, cinnamon, and salt together in a medium bowl. Add oil and eggs; mix well until just combined. Fold in zucchini and walnuts until they are evenly distributed; pour cake batter into the prepared pan.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, about 50 to 60 minutes. Cool cake completely before frosting with your favorite frosting.