Preheat oven to 350°F. Lightly grease a 9" square pan or line pan with parchment paper.
Whisk together sugar, cocoa powder, espresso powder, baking powder, salt, and flour; set aside.
Combine the zucchini, melted butter, eggs, and vanilla in work bowl of a food processor; process until smooth.
Add dry ingredients to zucchini / egg mixture and process briefly, just until combined.
Add chocolate chips and pulse several times to break up chips.
Pour the batter into the prepared pan.
Bake brownies for 25 to 30 minutes, or until a toothpick inserted into the center comes out clean, or with a few moist crumbs clinging to it; you shouldn’t see any sign of wet batter. Remove the brownies from the oven, and allow them to cool completely before frosting.
To make frosting: Combine the chocolate chips and milk or cream in a microwave-safe bowl or small saucepan.
Heat until the milk is steaming, and the chips are soft. Remove from the heat, and stir until smooth.
Spread the frosting on brownies, sprinkle with flaky sea salt if desired.
Place brownies in the refrigerator for an hour or so, to set. Store at room temperature, covered, for several days.
