Step 1: Prep Your Pan and Oven
Preheat your oven to 350°F (175°C). Grease a 9×13-inch baking pan or two 9×5-inch loaf pans generously with butter or non-stick spray. For extra insurance against sticking, line the bottom with parchment paper and grease the parchment too. This step makes removing the cake so much easier later!

Step 2: Shred the Zucchini
Wash your zucchini but don’t peel them—the skin contains nutrients and won’t be noticeable in the finished cake. Using the large holes of a box grater or the shredding blade of your food processor, shred the zucchini. You should end up with about 2 cups. There’s no need to squeeze out the moisture—all that liquid is what makes this cake incredibly moist! If your zucchini is very large with big seeds, you can scrape those out first, but it’s not necessary for most medium-sized zucchini.

Step 3: Mix the Wet Ingredients
In a large mixing bowl, whisk together the eggs, vegetable oil, granulated sugar, and vanilla extract. Beat vigorously for about a minute until the mixture is smooth and the sugar begins to dissolve. The mixture should look lighter in color and slightly thickened. This is the base that will give your cake structure and moisture.

Step 4: Add the Zucchini
Fold the shredded zucchini into the wet mixture using a rubber spatula. Stir until it’s evenly distributed throughout. The mixture will look a bit strange with all those green flecks, but trust the process—they completely disappear during baking!

Step 5: Combine the Dry Ingredients
In a separate large bowl, whisk together the flour, cocoa powder, baking soda, baking powder, salt, and cinnamon. Whisk for at least 30 seconds to ensure everything is evenly distributed and there are no cocoa powder lumps. This step is crucial for even rising and consistent chocolate flavor throughout the cake.

Step 6: Combine Wet and Dry
Pour the dry ingredients into the bowl with the wet ingredients. Using a rubber spatula or wooden spoon, fold the mixture together gently until just combined. Don’t overmix—stop as soon as you no longer see dry flour streaks. The batter will be thick and quite dark. If using chocolate chips, fold them in now, reserving a few tablespoons to sprinkle on top if desired.

Step 7: Bake
Pour the batter into your prepared pan(s), spreading it evenly with a spatula. If making loaf cakes, divide the batter evenly between both pans. Sprinkle reserved chocolate chips on top if using. Bake for 35-40 minutes for a 9×13-inch pan, or 45-55 minutes for loaf pans. The cake is done when a toothpick inserted in the center comes out with just a few moist crumbs (not wet batter). The top should look set and spring back lightly when touched. Don’t overbake or the cake will be dry!

Step 8: Cool Completely
Remove the cake from the oven and let it cool in the pan on a wire rack for 15 minutes. This allows the structure to set so the cake won’t fall apart when you remove it. After 15 minutes, you can either leave it in the pan to cool completely (easiest for sheet cakes) or carefully turn loaves out onto the cooling rack. Let cool completely before frosting—at least 1 hour.

Step 9: Make the Frosting (if using)
In a medium bowl, beat the softened butter until creamy. Add the cocoa powder and beat until combined. Gradually add the powdered sugar, alternating with splashes of milk, until you reach a spreadable consistency. Add vanilla and salt. Beat on high speed for 2-3 minutes until light and fluffy. Spread over the cooled cake.

Step 10: Slice and Serve
Cut into squares or slices and serve! This cake is delicious at room temperature, slightly warm from the oven, or even chilled. Store any leftovers covered at room temperature or in the refrigerator.