Preheat your oven to 350°F (175°C). Grease a 9x5-inch loaf pan or line it with parchment paper.
In a large mixing bowl, combine the grated zucchini, granulated sugar, brown sugar, vegetable oil, sour cream, eggs, and vanilla extract. Mix until well combined.
In another bowl, whisk together the flour, cocoa powder, baking soda, baking powder, salt, and cinnamon.
Gradually add the dry ingredients to the wet ingredients, stirring until just combined. Be careful not to overmix.
Fold in the chocolate chips and nuts (if using) until evenly distributed.
Pour the batter into the prepared loaf pan and smooth the top with a spatula.
Bake for 50-60 minutes, or until a toothpick inserted into the center comes out clean. If the top is browning too quickly, cover it loosely with aluminum foil during the last 15 minutes of baking.
Allow the bread to cool in the pan for 10 minutes, then transfer it to a wire rack to cool completely before slicing.