Preheat the oven to 350F. Grease a 9x5-inch loaf pan with butter or baking spray.
In a large mixing bowl, whisk together the flour, sugar, cocoa powder, baking powder, baking soda, cinnamon, espresso powder, and salt.
In a medium mixing bowl, whisk together the melted butter, oil, eggs, and vanilla. Add to the flour mixture, along with the zucchini, and chocolate chips. Stir together until combined. (The batter will be very thick). Scoop the batter into the prepared pan and spread into an even layer.
Bake for 60 to 70 minutes or until a toothpick inserted into the center comes out with a few crumbs. Let cool in the pan on a wire rack for 20 minutes. Remove the bread and cool completely on the rack or until slightly warm before slicing and serving. The bread can be wrapped and stored at room temperature for up to 4 days. Individually wrapped slices freeze great, too!
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Notes
Use room-temperature eggs so they are easier to incorporate into the batter and won't resolidify the melted butter. Set them out 30 minutes to an hour before you start baking. If you forget, bring them to room temperature fast by placing them in a large bowl of warm water for 5 minutes.
Use a scale to measure your flour. This will keep you from accidentally using too much flour. If you don't have a scale, fluff your flour with a spoon and then spoon it into the measuring cup before leveling it off with a knife.
Don't overmix the batter. You'll end up with a tough loaf instead of a soft, moist loaf.
Use a light-colored loaf pan to prevent the edges and bottom of the chocolate zucchini bread from browning too much.
Add nuts. Replace some of the chocolate chips with a handful of chopped nuts. Toasted walnuts, pecans, and hazelnuts are great add-ins.
For a pretty finishing touch: Sprinkle a few chocolate chips on top of the batter before baking.