In a large bowl, whisk together melted butter, sugar, vanilla, and salt.
Add the eggs, one at a time, whisking well after each addition.
Whisk in the cocoa powder and flour, mixing until just blended.
Fold in the grated zucchini and 1 1/2 cups chocolate chips.
Line a 9-inch square pan with foil, hanging it over the edges.
Spray foil with non-stick spray.
Spread batter evenly into the prepared pan.
Bake at 350 degrees for 45-50 minutes, or until a toothpick inserted near the center comes out mostly clean, with a few moist crumbs.
Remove to a wire rack to cool completely.
Meanwhile, prepare frosting.