Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners or grease it with non-stick spray.

In a medium bowl, combine the grated zucchini, vegetable oil, eggs, and vanilla extract. Mix well until fully combined.

In a separate large bowl, whisk together the all-purpose flour, cocoa powder, baking soda, baking powder, salt, granulated sugar, and brown sugar.

Pour the wet ingredients into the dry ingredients and stir until just combined. Avoid overmixing.

Fold in the semi-sweet chocolate chips and chopped walnuts, if using.

Scoop the batter into the prepared muffin tin, filling each cup about 3/4 full.

Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean.

Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.