For the cake:
- 1 cup plus 2 tablespoons (143 grams) all-purpose flour, measured correctly
- 1/2 cup (43 grams) unsweetened cocoa, measured correctly
- 1 cup (200 grams) light brown sugar
- 1 teaspoon salt
- 1 teaspoon baking soda
- 4 tablespoons (57 grams) unsalted butter, melted
- 1/4 cup fresh vegetable oil
- 1/2 cup (120 grams) full fat sour cream or plain whole milk yogurt, at room temperature
- 2 large eggs plus 1 egg yolk, at room temperature
- 2 teaspoons vanilla extract
- 1 cup (136 grams) shredded zucchini, from about 1 medium zucchini
- 1 cup (170 grams) semisweet chocolate chips
For the icing:
- 1/2 cup (63 grams) powdered sugar, sifted
- 1 tablespoon milk
Make the cake:
Preheat the oven to 350°Line a 8 by 8-inch metal baking pan with parchment paper then spray with nonstick cooking spray. If using a glass pan, heat the oven to 325°F.
In a large bowl, whisk together the flour, cocoa, sugar, salt, and baking soda.
In a small bowl whisk together the melted butter, oil, sour cream, eggs, egg yolk, and vanilla extract.
Make a well in the center of the dry ingredients and pour in the wet ingredients. Stir until a few streaks of flour remain. Squeeze out any additional moisture from the shredded zucchini before adding to the batter. See recipe post for tips on reducing excess moisture, which will create a gummy cake texture. Add in the chocolate chips. Stir until just barely combined.
Pour the batter into the prepared pan and bake for 30 minutes (35 to 40 minutes if using glass) or until a cake tester comes out clean. If your cake sinks slightly in the middle, that's normal because it's so moist and rich!
Let cool completely.
Make the frosting:
In a medium bowl stir together the sugar and milk with a fork until smooth and thick but pourable. Drizzle all over the cake. Let set before serving.
Cake can be stored in an airtight container at room temperature for up to 4 days.