A rich, ultra-moist chocolate cake made with sourdough discard and finely grated zucchini, finished with a soft, swoopy chocolate frosting. This easy 9×13 sheet cake bakes up tender, deeply chocolatey, and perfectly balanced with no visible zucchini.
Ingredients You’ll Need
Cake Batter
- ¾ cup (150g) granulated sugar
- ¾ cup (150g) brown sugar
- ⅔ cup (160ml) neutral oil
- 2 large eggs
- ½ cup (120g) sourdough discard recent, not overly acidic
- ½ cup (120g) sour cream
- 1 tsp vanilla extract
- ½ cup (50g) unsweetened natural cocoa powder
- 2 tbsp (10–12g) black cocoa powder optional, for deeper color
- 1⅔ cups (205–210g) all-purpose flour
- 1 tsp baking soda
- ½ tsp baking powder
- ¾ tsp salt
- 1-2 tsp instant espresso powder
- 1½ to 1¾ cups (200–210g) finely grated zucchini do not blot
- ¾ to 1 cup chocolate chips
For the Chocolate Frosting
- ¾ cup (170g) unsalted butter, softened
- ¼ cup (25g) cocoa powder
- 2½ to 3 cups (300–360g) powdered sugar
- 2-3 tbsp heavy cream
- 1 tsp vanilla extract
- Pinch of salt
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How to Make Chocolate Zucchini Sheet Cake
1. Preheat the oven to 350°F. Lightly grease a 9×13-inch metal baking pan, or line with parchment paper, leaving overhang on the sides for easy removal.
2. Grate the zucchini using the large holes of a box grater. Do not blot or squeeze dry unless it is extremely watery.
3. In a large bowl, whisk together the granulated sugar, brown sugar, oil, eggs, sourdough discard, sour cream, and vanilla until smooth and fully combined.
4. Add the cocoa powder (and black cocoa powder, if using) to the wet ingredients and whisk until smooth. Let the mixture sit for 1–2 minutes to allow the cocoa to hydrate.
5. In a separate bowl, whisk together the flour, baking soda, baking powder, salt, and espresso powder.
6. Add the dry ingredients to the wet ingredients and gently fold until just combined and no dry streaks remain.
7. Fold in the grated zucchini and chocolate chips until evenly distributed. The batter will be thick.
8. Pour the batter into the prepared pan and spread evenly.
9. Bake for 27–32 minutes, or until a toothpick inserted into the center comes out with moist crumbs and the internal temperature reaches about 198–200°F.
10. Allow the cake to cool completely in the pan or transfer to a board. Cover and let rest for several hours or overnight for best texture.
11. To make the frosting, beat the softened butter and cocoa powder together until smooth.
12. Add the powdered sugar, vanilla, salt, and 2 tablespoons of heavy cream. Beat until smooth and creamy, adding more cream as needed until soft and spreadable.
13. Spread the frosting over the cooled cake using an offset spatula, creating soft swoops.
Sourdough Discard Chocolate Zucchini Sheet Cake with Chocolate Frosting
Equipment
- Digital thermometer optional but recommended
Ingredients
Cake Batter
- ¾ cup (150g) granulated sugar
- ¾ cup (150g) brown sugar
- ⅔ cup (160ml) neutral oil
- 2 large eggs
- ½ cup (120g) sourdough discard recent, not overly acidic
- ½ cup (120g) sour cream
- 1 tsp vanilla extract
- ½ cup (50g) unsweetened natural cocoa powder
- 2 tbsp (10–12g) black cocoa powder optional, for deeper color
- 1⅔ cups (205–210g) all-purpose flour
- 1 tsp baking soda
- ½ tsp baking powder
- ¾ tsp salt
- 1-2 tsp instant espresso powder
- 1½ to 1¾ cups (200–210g) finely grated zucchini do not blot
- ¾ to 1 cup chocolate chips
For the Chocolate Frosting
- ¾ cup (170g) unsalted butter, softened
- ¼ cup (25g) cocoa powder
- 2½ to 3 cups (300–360g) powdered sugar
- 2-3 tbsp heavy cream
- 1 tsp vanilla extract
- Pinch of salt
Instructions
- Preheat the oven to 350°F. Lightly grease a 9×13-inch metal baking pan, or line with parchment paper, leaving overhang on the sides for easy removal.
- Grate the zucchini using the large holes of a box grater. Do not blot or squeeze dry unless it is extremely watery.
- In a large bowl, whisk together the granulated sugar, brown sugar, oil, eggs, sourdough discard, sour cream, and vanilla until smooth and fully combined.
- Add the cocoa powder (and black cocoa powder, if using) to the wet ingredients and whisk until smooth. Let the mixture sit for 1–2 minutes to allow the cocoa to hydrate.
- In a separate bowl, whisk together the flour, baking soda, baking powder, salt, and espresso powder.
- Add the dry ingredients to the wet ingredients and gently fold until just combined and no dry streaks remain.
- Fold in the grated zucchini and chocolate chips until evenly distributed. The batter will be thick.
- Pour the batter into the prepared pan and spread evenly.
- Bake for 27–32 minutes, or until a toothpick inserted into the center comes out with moist crumbs and the internal temperature reaches about 198–200°F.
- Allow the cake to cool completely in the pan or transfer to a board. Cover and let rest for several hours or overnight for best texture.
- To make the frosting, beat the softened butter and cocoa powder together until smooth.
- Add the powdered sugar, vanilla, salt, and 2 tablespoons of heavy cream. Beat until smooth and creamy, adding more cream as needed until soft and spreadable.
- Spread the frosting over the cooled cake using an offset spatula, creating soft swoops.
Notes
- Use a recent sourdough discard for the best flavor and structure.
- Do not blot the zucchini unless it is extremely watery. Fresh, in-season zucchini tends to have higher water content and may release more moisture when grated. If your zucchini feels very wet or watery after grating, gently pat it once with a paper towel, but do not squeeze it dry.
- Out-of-season or smaller zucchini are often less watery and can be used as-is without blotting. The added moisture from the zucchini is essential for a soft, tender cake.
- Grating on the larger holes of a box grater helps the zucchini melt into the cake without visible texture while still providing moisture.
- Do not overbake. Pulling the cake around 198–200°F ensures a soft, moist crumb.
- Letting the cake rest overnight after baking improves both texture and flavor.
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