Chocolate-Zucchini Cakes with Walnuts

INGREDIENTS

• 1/2 cup (1 stick) unsalted butter, melted and cooled
• 1 cup sugar
• 1 large egg
• 24 walnut halves
• 1 cup all-purpose flour
• 1 cup finely grated zucchini (from 1 medium zucchini)
• 1/4 cup unsweetened cocoa powder
• 1/2 cup (3 ounces) bittersweet chocolate, chopped, or chocolate chips
• 3 tablespoons sour cream
• 1/2 teaspoon coarse salt
• 1/2 teaspoon pure vanilla extract
• Nonstick cooking spray