Heat oven to 425°F. Microwave chocolate and butter in large microwaveable bowl on HIGH 2 min. or until butter is melted. Stir with whisk until chocolate is completely melted.
Beat whole eggs, yolks, and sugar in a mixer until light in color and thick. Stir in flour. Gradually beat into chocolate mixture.
Line12 (2-1/2-inch) muffin cups with paper liners; spray with cooking spray. Fill with batter.