Preheat oven to 350℉. Butter a 9x13-inch metal pan and line it with parchment paper.
In a medium saucepan over medium-low heat, melt together 2 cups chocolate chips and butter while whisking constantly. Do not boil. Once smooth, remove from heat and set aside to cool for 15 minutes.
In a small bowl, whisk together flour, cocoa powder, Clabber Girl® Baking Powder, baking soda, and salt.
In a large bowl, whisk together eggs, sugar, oil, and vanilla until blended. Whisk in the cooled chocolate mixture.
Add dry ingredients to the large bowl of wet ingredients and stir until well combined.
Fold in the zucchini until well incorporated. Pour the batter into the prepared baking pan. Sprinkle the remaining ½ cup chocolate chips over the top.
Bake at 350℉ for 35-40 minutes. For fudgy brownies, bake for 35-37 minutes or until a toothpick comes out mostly clean. For firmer brownies, bake for 40 minutes or until a toothpick comes out clean. Cool in the pan then cut into bars.