Preheat the oven to 325°F. Grease an 8x8-inch baking dish and line it with parchment paper.

In a small bowl, stir together the flaxseed and water and set aside for 5 minutes to thicken.

In a medium bowl, whisk together the flour, sugar, cocoa powder, baking powder, and salt.

In a large bowl, whisk together the coconut oil, vanilla, and flaxseed mixture. Stir in the zucchini.

Add the dry ingredients to the zucchini mixture and stir to combine as best you can. Let rest for 1 minute and stir again. The mixture will be thick and dry at first, but it will become smoother and wetter as the zucchini releases moisture into the batter. Stir until there are no dry areas remaining, then fold in the chocolate chips.

Pour the batter into the prepared pan and use a spatula to smooth the top. Sprinkle with more chocolate chips and flaky sea salt, if using. Bake for 45 to 50 minutes, or until the brownies are set in the middle and a toothpick inserted into the center comes out clean.

Let cool for at least an hour before slicing.