Prep the zucchini: Shred it using a box grater or food processor. Toss with ½ teaspoon salt and let it sit in a fine-mesh strainer over a bowl for 10 minutes. Then grab handfuls and squeeze firmly over the sink — you'll be surprised how much liquid comes out. This step is non-negotiable for a loaf that bakes up tender, not wet.
Preheat and prep the pan: Preheat your oven to 350°F. Line a 9x5-inch loaf pan with parchment paper, leaving a 2-inch overhang on the long sides. Lightly grease the ends with butter or oil.
Mix the dry ingredients: In a medium bowl, whisk together the flour, cocoa powder, baking soda, baking powder, and salt. Set aside.
Mix the wet ingredients: In a large bowl, whisk the eggs, brown sugar, and granulated sugar together until smooth and slightly thickened — about 30 seconds. Stream in the oil and melted butter while whisking. Add the buttermilk and vanilla, and whisk until combined.
Add the melted chocolate: Pour the slightly cooled melted chocolate into the wet ingredients and whisk until the batter is smooth and glossy. This is where the loaf starts to feel like something special.
Combine dry and wet: Add the dry ingredients to the wet mixture and fold gently with a rubber spatula until just combined — a few streaks of flour are fine. Overmixing develops gluten and makes the bread tough.
Fold in the zucchini: Add the squeezed, dry zucchini to the batter and fold until evenly distributed. The batter will be thick and luscious. Fold in the optional chocolate chips if using.
Bake: Scrape the batter into the prepared pan and smooth the top. Bake for 50-60 minutes, or until a skewer inserted in the center comes out with a few moist crumbs attached (not raw batter). If the top is browning too quickly at the 40-minute mark, tent loosely with foil.
Cool completely: Let the loaf cool in the pan on a wire rack for 15 minutes. Then lift it out using the parchment overhang and let it cool completely before slicing — at least 1 hour. I know the smell is intoxicating. I know. But slicing it warm will give you a crumbly mess. Trust the process.