Preheat your oven to 325°F and grease a 9x13 inch baking dish or similar size with butter or cooking spray. While the oven heats, whisk together the flour, sifted cocoa powder, salt, baking soda, and espresso powder (if using) in a medium bowl. Setting up your mise en place now means you'll work efficiently once you start mixing the wet ingredients.

In a large bowl, whisk together the melted butter, sugar, sourdough starter, egg, egg yolk, and vanilla extract until well combined and slightly lighter in appearance. The sourdough starter adds tanginess and depth that enhances the chocolate flavor, so make sure it's fully incorporated. This mixture is the foundation for moisture and flavor, so take your time to whisk thoroughly.

Add the dry ingredient mixture from Step 1 to the wet mixture from Step 2, then fold in the shredded zucchini until just combined—don't overmix, as this can make the brownies tough. Let the batter rest for 10 to 15 minutes at room temperature; I find this resting period allows the zucchini to release moisture and the cocoa to fully hydrate, which creates a more fudgy texture. After resting, stir the batter once more and fold in the chocolate chips.

Pour the batter into your prepared baking dish and spread it evenly. Bake at 325°F for 35 to 42 minutes, until a toothpick inserted into the center comes out with just a few moist crumbs—remember, these are meant to be fudgy, not dry. Once baked, let the brownies cool completely in the pan before cutting into squares, then sprinkle with flaky sea salt over the top for a subtle contrast to the rich chocolate.