Instructions

• Preheat the oven to 350°F (175°C). Grease a 9×13-inch baking pan with nonstick cooking spray and set it aside.
• In a medium bowl, whisk together the all-purpose flour, the sifted unsweetened cocoa powder (for the batter), the baking soda, and the salt. Set the dry mixture aside.
• In a large bowl, combine the brown sugar, vegetable (or canola or avocado) oil, the cooled melted butter, the room-temperature buttermilk, the room-temperature eggs, and the vanilla extract. Whisk until the wet ingredients are fully combined.
• Add the dry ingredients to the wet ingredients and stir gently until just combined—do not overmix. It is fine if a few streaks of flour remain.
• Prepare the shredded zucchini by squeezing it in a clean kitchen towel or between paper towels to remove excess liquid; leave the zucchini moist but not dripping. Fold the squeezed zucchini and the chocolate chips into the batter until just distributed.
• Pour the batter into the prepared 9×13-inch pan and smooth the top with a spatula.
• Bake for 25–32 minutes, or until a toothpick inserted into the center of the cake comes out clean or with a few moist crumbs. Remove the cake from the oven and let it cool completely in the pan on a wire rack before frosting.
• While the cake cools, make the frosting: in the bowl of a stand mixer or using a hand mixer, beat the room-temperature unsalted butter until smooth, about 1 minute.
• Sift together the confectioner’s sugar and the unsweetened cocoa powder for the frosting. With the mixer on low speed, add the sifted sugar–cocoa mixture to the butter in batches, and slowly add the milk as you beat until the frosting reaches a smooth, spreadable consistency.
• Add the vanilla extract and a pinch of salt, scrape down the sides of the bowl, and beat until the frosting is smooth and fluffy.
• Spread the frosting evenly over the completely cooled cake. Cut into squares and serve.