Preheat oven to 350F. Butter a 9x5" loaf pan, then line with parchment paper so it has "sleeves" on the edges for easy removal.

In a large bowl, combine the flour, cocoa powder, baking powder, baking soda, cinnamon, salt, and nutmeg. Mix very well. Set aside.

In a separate bowl, vigorously whisk together the eggs, yogurt, sugar, olive oil, honey, and vanilla for 2 minutes.

Add the flour mixture and fold together JUST until combined with a spatula. It's okay if there's still some dry spots.

Add the finely grated zucchini, half of the chopped chocolate, and (optional) chopped walnuts. Fold together once again just until new additions are evenly distributed and there's no more dry flour spots.

Transfer the batter to the prepared loaf pan. Top with the remaining chopped chocolate.

Bake in the middle oven rack for 55 to 60 minutes, until a toothpick inserted into the center comes out clean or only with a few moist crumbs. Let rest in the pan for 20 minutes before slicing.