Preheat the oven to 350°F and grease a 9×13-inch pan; line with parchment with an overhang for easy removal.

Grate zucchini on the large holes of a box grater or in a food processor and measure 2 cups packed; if very wet, press briefly in a fine-mesh sieve to remove excess water.

In a medium bowl whisk together flour, cocoa powder, baking powder, baking soda, salt, and cinnamon.

In a large bowl whisk the eggs, then add oil, granulated sugar, and brown sugar and whisk until smooth; stir in vanilla.

Fold the grated zucchini into the wet mixture, then fold in the dry ingredients until just combined; fold in 3/4 cup chocolate chips and reserve the rest for topping.

Spread the batter evenly in the prepared pan and bake at 350°F for 22–28 minutes, rotating halfway; a toothpick should come out with moist crumbs but not wet batter.

Cool the pan on a wire rack for about 20 minutes before frosting.

Gently melt peanut butter, chopped chocolate, and butter together in a small saucepan over low heat or in 20-second microwave bursts, stirring until smooth.

Off heat, whisk in powdered sugar and add 1 tablespoon milk at a time until spreadable; spread over warm but not hot brownies and sprinkle remaining chocolate chips.

Let frosting set about 30–45 minutes, then lift brownies from the pan using the parchment and cut into 12 squares.