1. Preheat the oven to 350 degrees F. Spray a 9 by 4-inch loaf pan or 3 mini loaf pans with cooking spray. Set aside.
2. Shred the zucchini lengthwise, turning to get as much flesh and as little seed as possible. When you get down to the seeds, stop shredding and discard the seeds. You should have a generous cup of zucchini. Squeeze the zucchini by hand to remove excess moisture.
3. Whisk together the buttermilk, vegetable oil, vanilla extract and eggs in a large liquid measuring cup. Stir in the zucchini.
4. Combine the flour, sugar, cocoa, baking powder, baking soda and salt in a medium bowl.
5. Stir the wet ingredients into the dry ingredients until just incorporated, take care not to over mix. Fold in 3/4 cup of the nuts and all of the chocolate.
6. Transfer the batter to the prepared loaf pan(s), scatter the remaining 1/4 cup nuts over the top. Tap the bottom on the work surface to distribute the batter in the pan evenly. Bake until a toothpick inserted in the center comes out clean, 55 minutes to 1 hour for the large pan and 35 to 40 minutes for the minis. Transfer the pan(s) to a wire rack to cool 30 minutes. Turn the bread out and let cool completely on the rack. Save in an airtight container for up to 1 week.