Preheat oven to 325 degrees F. Line twenty-four 2-1/2-inch muffin cups with double or single layer of paper bake cups or lightly coat with nonstick cooking spray; set aside. 

In a large bowl stir together zucchini, eggs, granulated sugar, oil, and vanilla. Add flour, cocoa powder, baking soda, salt, baking powder, and, if desired, chocolate pieces; stir until combined. Spoon batter into prepared pans, filling cups about half full. Bake about 25 minutes or until a wooden toothpick inserted near centers comes out clean. Cool in pan on wire racks for 5 minutes. Remove from cups. Cool completely on racks. Frost with Peanut Butter Frosting. 

Makes: 24 cupcakes 

Peanut Butter Frosting: In a medium mixing bowl beat 1/2 cup peanut butter, 1/3 cup softened butter, 1 tablespoon milk, and 1/2 teaspoon vanilla with an electric mixer on medium speed until smooth. Gradually add 1-1/2 cups powdered sugar, beating until combined. If necessary, stir in 1 to 2 teaspoons additional milk until frosting reaches desired consistency. Makes about 1-1/2cups. 

Test Kitchen Tip: If you only have 12 muffin cups, cover and refrigerate half of the batter while the first batch bakes.