1/2 cup unsalted butter, melted and cooled
1 cup sugar
1/2 teaspoon salt
1 large egg
1/2 teaspoon pure vanilla extract
1 cup finely grated zucchini (from 1 medium zucchini)
3 tablespoons sour cream
1 cup all-purpose flour
1/4 cup unsweetened cocoa powder
1/3 cup (3 ounces) bittersweet chocolate, chopped, or chocolate chips
nonstick cooking spray
24 walnut halves
Preheat oven to 350 degrees.
In a large bowl, stir together butter, sugar, salt, and egg until combined.
Add vanilla, zucchini, and sour cream and stir until incorporated.
Sift flour and cocoa powder into bowl and stir until combined.
Stir in chocolate.
Spray two mini muffin pans or 1 cupcake pan with cooking spray. Fill each cup with 2 tablespoons batter for mini muffins, or 1/4 cup batter for cupcakes.
Top with a walnut.
Bake until a toothpick inserted in center comes out clean, 15 to 17 minutes. Let muffins cool slightly in pans on wire racks before serving.