Preheat oven to 325℉. Grease liberally a 9-by-5-inch loaf pan with soft butter and line it with a strip of parchment paper that hangs over the long sides to create a sling
In a large bowl, whisk together the eggs, brown sugar, buttermilk, melted butter, vanilla extract and salt until smooth and glossy. Add the grated zucchini and mix with a spatula until well incorporated
Using a fine-mesh strainer, sift the flour, cocoa powder, baking powder, baking soda and cinnamon directly into the bowl and stir with the spatula just until combined and no pockets of unincorporated flour remain. Do not overmix. Add all but 1 tablespoon of the chocolate chips to the batter and stir until evenly distributed
Pour the batter into the prepared pan. Sprinkle the top with the turbinado sugar and reserved chocolate chips, pressing them in slightly so they stick
Bake until the loaf puffs slightly and a skewer or cake tester inserted into the center comes out clean with just a few moist crumbs attached. You will have smudges of melted chocolate chips on your skewer! Bake for 75 to 80 minutes
Let the loaf cool in the pan for 15 minutes. Using the parchment paper, lift the loaf out of the pan, remove the parchment and let cool completely on a rack before slicing, about 2 hours