Preheat the oven to 325°F and place a rack in the center. Grease a 9×5-inch loaf pan and line with parchment, leaving a 1–2 inch overhang.

Shred zucchini on the coarse side of a box grater, place shreds in a clean towel, and squeeze gently to remove excess water until moist but not dripping.

Whisk together flour, Dutch-process cocoa, baking powder, baking soda, and salt in a medium bowl to remove lumps.

In a large bowl whisk oil, eggs, vanilla, and brown sugar until smooth, then add the dry ingredients and fold gently until just combined.

Fold in the shredded zucchini and semisweet chocolate chips (toss chips in 1 tablespoon flour first if using large chips). If batter looks dry, stir in up to 1/2 cup milk a tablespoon at a time; the batter should be thick and scoopable.

Transfer batter to the prepared pan, smooth the top, and bake at 325°F for 50–65 minutes. Check at 50 minutes with a thin skewer—the center should have moist crumbs but no raw batter. Tent with foil if top browns too fast.

Cool the loaf in the pan on a wire rack for 15–20 minutes, then lift out with parchment and cool completely before slicing for clean cuts. Store cooled loaf wrapped at room temperature for up to 3 days or freeze wrapped for up to 3 months.