Preheat the oven to 350 degrees F. Butter/grease a 9×5 loaf pan, dust with cocoa powder, tapping out an excess, and set aside until ready to use.
In a large bowl, whisk together the dry ingredients(the flour, cocoa powder, baking soda, and salt).
In a separate large mixing bowl, combine all the wet ingredients, except the grated zucchini (the sugar, melted butter, egg, egg yolk, and vanilla). Add the zucchini and fold with a wooden spoon or rubber spatula until just incorporated.
Add the flour mixture to the zucchini mixture 1/3 at a time, folding until combined between each addition.
Finally, add the chocolate chunks(or chocolate chips) and fold to mix.
Spoon finished batter into your prepared loaf pan, and place it in the oven.
Bake for 50 minutes, or until a toothpick inserted into the center of the loaf comes out clean.
Allow the bread to cool in the pan for at least 10 minutes before turning it out onto a wire rack to finish cooling. Cool to room temperature, then slice and serve.
Enjoy!