Preheat the oven to 350 degrees F. Line two baking sheets with parchment paper and set them aside.

Grate the zucchini on the large holes of a box grater. Bundle the shreds in a clean kitchen towel and squeeze hard over the sink until the towel stops dripping. Measure 1 cup of the pressed zucchini and set it aside.

Chop the dark chocolate bar on a cutting board using a sharp knife. Aim for a mix of small shards and larger rough chunks, about 1/4 to 1/2 inch. The uneven sizes are what give you both big melty pools and little flecks throughout the baked cookie. Set aside.

In a medium bowl, whisk together the flour, cocoa powder, baking soda, baking powder, and fine salt until the cocoa is fully incorporated with no streaks. Set aside.

In a large bowl, beat the softened butter, brown sugar, and granulated sugar together on medium speed for 2 minutes until the mixture is light and fluffy. Add the egg and vanilla and beat for another 30 seconds.

Pour the flour and cocoa mixture into the butter mixture and stir with a spatula until just combined. The dough will look dark and fudgy. Fold in the squeezed zucchini and the chopped chocolate chunks, stirring until everything is evenly distributed.

Scoop rounded tablespoons of dough onto the prepared baking sheets, spacing them about 2 inches apart. Give each mound a gentle press to flatten slightly since the dough will not spread dramatically on its own.

Bake for 10 to 12 minutes. The edges should look set and the tops should appear dry and just barely crackled. The centers will look slightly underdone, but that is correct. They firm up to a fudgy texture as they cool. Do not wait until they look fully baked in the oven or they will dry out.

Pull the pans from the oven and immediately sprinkle a pinch of flaky sea salt over each cookie while they are still hot. This is when the salt sticks properly. Let the cookies rest on the pan for 5 minutes, then transfer to a wire rack to finish cooling.