Step 1: Prepare and Preheat
Preheat your oven to 350°F (175°C). Grease and flour a 9×13-inch baking pan, or line with parchment paper for easy removal. If making a layer cake, prepare two 9-inch round pans. Set aside.

Step 2: Prepare the Zucchini
Wash zucchini and trim off the ends (no need to peel unless skin is very thick). Using a box grater or food processor, shred the zucchini. You should have about 2 cups loosely packed. Place shredded zucchini in a clean kitchen towel or several layers of paper towels. Gently squeeze to remove some excess moisture – you want to remove about half the liquid, but don’t squeeze bone-dry. The zucchini should still be moist. Set aside.

Step 3: Mix Dry Ingredients
In a large mixing bowl, whisk together flour, cocoa powder, sugar, baking soda, baking powder, salt, and cinnamon (if using). Whisk thoroughly to remove any lumps, especially in the cocoa powder. If you have a sifter, you can sift the cocoa powder first for extra smoothness.

Step 4: Mix Wet Ingredients
In a separate medium bowl, whisk together the oil, eggs, vanilla extract, and buttermilk until well combined and slightly frothy. The mixture should be smooth and uniform.

Step 5: Combine Everything
Make a well in the center of the dry ingredients and pour in the wet ingredients. Using a wooden spoon or rubber spatula, gently stir until just combined – don’t overmix! A few small lumps are fine. Overmixing develops gluten and creates a tough cake. Gently fold in the shredded zucchini until evenly distributed throughout the batter. If using chocolate chips or nuts, fold those in now. The batter will be thick and somewhat lumpy – this is perfect!

Step 6: Bake the Cake
Pour batter into your prepared pan, spreading evenly with a spatula. For a 9×13-inch pan, bake for 35-40 minutes. For two 9-inch round pans, bake for 28-32 minutes. The cake is done when a toothpick inserted into the center comes out with just a few moist crumbs (not wet batter, but not completely clean). The top should spring back lightly when touched. Don’t overbake or the cake will be dry!

Step 7: Cool
Remove from oven and let the cake cool in the pan on a wire rack for 15 minutes. If making a layer cake, after 15 minutes carefully invert onto the wire rack, remove pans, and let cool completely. For a sheet cake, you can either frost it in the pan or turn it out onto a serving platter once completely cool.

Step 8: Make Frosting (Optional)
If frosting your cake, wait until it’s completely cool (this is crucial!). In a large bowl, beat softened butter until creamy. Add cocoa powder and mix until combined. Gradually add powdered sugar, one cup at a time, alternating with splashes of milk. Beat until smooth, light, and fluffy, about 3 minutes. Add vanilla extract and salt. If frosting is too thick, add more milk one tablespoon at a time. If too thin, add more powdered sugar.

Step 9: Frost and Serve
Spread frosting evenly over the completely cooled cake using an offset spatula or butter knife. For a layer cake, place one layer on a serving plate, spread frosting on top, add second layer, then frost top and sides. Slice and enjoy! This cake is also wonderful served plain with just a dusting of powdered sugar or with a dollop of whipped cream.