Preheat oven to 350°F. Grease a 9x5-inch loaf pan.
Whisk flour, cocoa, baking soda, baking powder, salt, and cinnamon together in a bowl.
In another bowl, beat oil, granulated sugar, and brown sugar until blended. Add eggs one at a time, then vanilla and sour cream.
Stir in the dry ingredients until just combined.
Fold in grated zucchini and chocolate chips. The batter will be thick, don’t worry.
Spoon into the loaf pan and smooth the top.
Bake 55–65 minutes until a toothpick in the center comes out mostly clean.
Cool for 15 minutes in the pan, then transfer to a rack to cool completely.