Chocolate Zucchini Cake Recipe: Moist 9×13 Cake with Chocolate Glaze

By Aquarium Guides Editorial Team · Updated November 10, 2026

Last Updated: November 10, 2026

Chocolate zucchini cake is a moist, tender sheet cake in which shredded zucchini disappears into the batter and keeps the crumb soft. The zucchini adds moisture rather than flavor, so the cake tastes like chocolate. This one-bowl recipe uses cocoa powder, oil and buttermilk, bakes in a 9×13-inch pan and is topped with a simple chocolate glaze. It is a good way to use summer garden zucchini, and the cake stays moist for days.

Ingredients

This makes one 9×13-inch cake and serves 12 to 16.

For the cake

• 2 1/2 cups (315 g) all-purpose flour
• 1/2 cup (45 g) unsweetened natural cocoa powder
• 1 1/2 teaspoons baking soda
• 1/2 teaspoon baking powder
• 1 teaspoon fine sea salt
• 1 teaspoon ground cinnamon
• 3/4 cup vegetable or canola oil
• 1 1/2 cups granulated sugar
• 1/2 cup light brown sugar, packed
• 3 large eggs, at room temperature
• 2 teaspoons vanilla extract
• 1/2 cup buttermilk (or milk with 1/2 tablespoon lemon juice)
• 2 1/2 cups (about 2 medium) finely shredded zucchini, unpeeled
• 1 cup semisweet chocolate chips, divided

For the chocolate glaze

• 3/4 cup semisweet chocolate chips
• 1/3 cup heavy cream
• 1 tablespoon unsalted butter
• 1 teaspoon light corn syrup (optional, for shine)

Instructions

1. Heat the oven to 350°F. Grease a 9×13-inch pan, line the bottom with parchment, and grease the parchment.
2. Shred the zucchini using the small holes of a box grater. Place it in a clean kitchen towel and gently squeeze out the visible excess liquid. Measure 2 1/2 cups of packed zucchini.
3. In a medium bowl, whisk together the flour, cocoa, baking soda, baking powder, salt and cinnamon.
4. In a large bowl, whisk the oil, both sugars, eggs and vanilla until smooth and slightly thickened, about 1 minute. Stir in the buttermilk.
5. Add the dry ingredients to the wet in two additions, stirring just until combined.
6. Fold in the shredded zucchini and 3/4 cup of the chocolate chips.
7. Pour the batter into the prepared pan, spread it evenly and sprinkle the remaining 1/4 cup chocolate chips on top.
8. Bake for 35 to 42 minutes, until a toothpick inserted in the center comes out with a few moist crumbs, not wet batter.
9. Cool the cake completely in the pan on a wire rack, about 1 hour.
10. For the glaze, heat the cream until steaming (not boiling), pour it over the chocolate chips, let it sit for 2 minutes, then whisk until smooth. Stir in the butter and corn syrup. Cool 5 to 10 minutes until slightly thickened, then pour over the cake and spread.

Why It Works

Oil makes a moist cake

Oil stays liquid at room temperature, so oil-based cakes feel soft and moist even after refrigeration. Butter-based cakes usually firm up when cold.

Zucchini adds moisture

Shredded zucchini is about 95 percent water, which releases as the cake bakes, producing a tender crumb. The shreds are small enough to blend in, and the dark batter hides the green flecks. Squeeze it lightly: you want to remove excess surface liquid but not to dry it out completely.

Natural cocoa and baking soda

Natural cocoa is acidic and reacts with baking soda for lift. If you use Dutch-process cocoa, replace the baking soda with 2 teaspoons of baking powder or add a small amount of extra acid such as buttermilk.

Room-temperature eggs

Eggs at room temperature blend evenly into the batter. If you forgot to take them out, place them in warm water for 5 minutes.

Do not overmix

Stir only until the flour disappears. Overmixing develops gluten and produces a rubbery cake.

Preparing Zucchini

Variations

• Cream cheese frosting: Swap the glaze for cream cheese frosting.
• Cupcakes: Fill lined muffin cups about two-thirds full and bake 18 to 22 minutes.
• Bundt cake: Bake in a well-greased 10-cup Bundt pan for 50 to 60 minutes.
• Nuts: Fold in 3/4 cup chopped walnuts or pecans.
• Espresso: Add 1 teaspoon instant espresso powder to the dry ingredients to deepen the chocolate flavor.
• Orange: Add 1 tablespoon orange zest to the wet ingredients.
• Layer cake: Divide the batter between two 9-inch pans and bake 28 to 32 minutes.
• Dairy-free: Use plant milk with a splash of lemon juice and dairy-free chocolate chips.

Troubleshooting

• Gummy center: Too much zucchini moisture or underbaking. Squeeze lightly and bake until a toothpick comes out clean or with moist crumbs.
• Sunken middle: Opening the oven too early or old leavening. Check expiration dates.
• Dry cake: Overbaking. Start testing at 35 minutes.
• Glaze too thin: Cool longer, or add a tablespoon of chocolate chips and re-warm.

Storage and Make-Ahead

• Room temperature: Cover and store for up to 3 days.
• Refrigerate: Keeps up to 5 days; bring slices to room temperature or microwave 10 seconds before serving.
• Freeze: Wrap unglazed cake (whole or in squares) tightly and freeze for up to 3 months. Thaw overnight and glaze before serving.
• Make ahead: Bake the cake a day in advance and glaze on the day you serve it.

Serving Ideas

Cut the cake into squares and serve at room temperature with a scoop of vanilla ice cream, a spoonful of whipped cream or a handful of fresh raspberries. For lunchbox treats, skip the glaze and dust with powdered sugar so slices stack without sticking. When zucchini is abundant, make two pans and freeze one unglazed for later.

FAQ

Can you taste the zucchini?

No. Zucchini has a mild flavor, and the cocoa and chocolate chips dominate.

Do I need to drain the zucchini?

Squeeze out excess surface liquid with a towel, but do not wring it dry. Some moisture contributes to the tender texture.

Can I use butter instead of oil?

You can substitute melted, cooled butter at an equal amount, though the crumb will be a little less moist, particularly after refrigeration.

Can I use whole wheat flour?

Replace up to half of the all-purpose flour with whole wheat. The cake will be denser and a bit nuttier.

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