Preheat oven to 350ºF. Line two loaf pans with parchment paper.
In a large bowl, whisk together flour, sugar, cocoa, baking powder, baking soda, and salt.
In a separate bowl, whisk together the zucchini, oil, sour cream, milk, eggs and vanilla extract. Add the flour mixture and fold together with a rubber spatula until 80% combined. Fold in chocolate (and now the batter should be uniform but not overmixed).
Divide batter between two loaf pans and bake in centre of oven until top feels springy when pressed, 40-50 minutes.