
Preheat the oven to 325°F. Line a light-colored metal 8 1/2 by 4 1/2-inch loaf pan** with a sheet of parchment paper, leaving an overhang, then spray with nonstick cooking spray.
In a large bowl, whisk together the flour, cocoa powder, sugar, baking soda, and salt.
In a medium bowl, whisk together the eggs, sour cream, oil, and vanilla until thoroughly combined.
Make a well in the center of the dry ingredients and stir the wet ingredients in with a rubber spatula until just combined. Fold in the zucchini and chocolate chips, reserving some to sprinkle on top. Pour batter into prepared pan, smoothing the top. Sprinkle with reserved chocolate chips.
Bake for about 70 minutes or until a digital thermometer registers at least 200°F. Alternatively, check that a toothpick inserted in the center comes out clean with just a few crumbs attached. Make sure melted chocolate chips aren’t mistaken for uncooked batter. Let the loaf cool in baking pan on wire rack for 15 minutes before removing to a wire rack to cool completely. Loaf can be stored well-wrapped at room temperature for 1 to 2 days or frozen for up to 1 month.