Combine the flour, Dutch-process cocoa powder, baking powder, baking soda, salt, and optional espresso powder in a medium bowl. Whisk thoroughly, breaking apart any lumps of cocoa powder.

In a separate large mixing bowl, whisk the eggs with the brown sugar and granulated sugar until the mixture looks smooth and evenly blended.

Pour in the oil, sour cream, and vanilla. Whisk again until completely incorporated.

Add the finely shredded zucchini and stir or fold until it is evenly distributed throughout the wet mixture.

Pour the dry ingredients into the wet ingredients. Using a rubber spatula, gently fold the two mixtures together. Stop while a few small streaks of flour are still visible.

Add 1 cup of the chocolate chips and continue folding just until the flour disappears and the chocolate chips are distributed throughout the batter. Avoid overmixing.

Divide the batter evenly among the 12 prepared muffin cups, filling each one almost to the top. Sprinkle some of the remaining chocolate chips over the batter, saving a small handful for after baking.

Check for doneness by inserting a toothpick into the center of a muffin. It should come out with a few moist crumbs attached. If you hit a melted chocolate chip, that's perfectly fine.

Remove the muffins from the oven and immediately press the reserved chocolate chips into the warm tops.

Allow the muffins to cool in the pan for about 5 minutes before transferring them to a wire cooling rack.