Preheat oven to 350°F (175°C). Grease a 9x5-inch loaf pan and line with parchment paper, leaving an overhang on the two long sides.

In a medium bowl, whisk together the flour, cocoa powder, baking soda, baking powder, salt, and cinnamon. Set aside.

In a large bowl using a hand mixer or stand mixer with the paddle attachment, cream the softened butter, granulated sugar, and brown sugar together on medium-high speed for 2-3 minutes, until light and fluffy.

Add the eggs one at a time, beating well after each addition. Scrape down the sides of the bowl. Mix in the sour cream and vanilla extract until combined.

With the mixer on low speed, gradually add the dry ingredient mixture to the wet ingredients. Mix only until just combined and no streaks of flour remain. Do not overmix.

Using a spatula, gently fold in the grated, drained zucchini and the 1 cup of chocolate chips until evenly distributed.

Pour the batter into the prepared loaf pan and smooth the top. Sprinkle with extra chocolate chips if desired.

Bake for 55-65 minutes, or until a wooden skewer inserted into the center comes out with a few moist crumbs attached (no wet batter).

Let the bread cool in the pan on a wire rack for 20 minutes. Then, use the parchment paper to lift the bread out and transfer it to the wire rack to cool completely before slicing.