Preheat the oven to 350°F (175°C). Grease a 9×13 inch sheet pan or line it with parchment paper.
In a large mixing bowl, whisk together the granulated sugar, brown sugar, and vegetable oil until well combined.
Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
Add the shredded zucchini to the wet mixture and stir until evenly distributed.
In a separate bowl, sift together the all-purpose flour, cocoa powder, baking powder, baking soda, and salt.
Gradually add the dry ingredients to the wet ingredients, stirring gently until just combined. Do not overmix.
Fold in the chocolate chips if using.
Pour the batter into the prepared sheet pan and spread it evenly.
Bake for 28-30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs but not wet batter.
Remove from the oven and allow to cool completely in the pan on a wire rack before cutting into 12 squares.