Preheat oven to 425 degrees F. Butter 4 (3/4-cup) custard cups or souffle dishes. Place on baking sheet.
Microwave chocolate and 1/2 cup butter in large microwavable bowl on HIGH 1 min. or until butter is melted. Stir with wire whisk until chocolate is completely melted. Stir in sugar until well blended. Whisk in eggs and egg yolks. Stir in flour. Divide batter between prepared custard cups.
Bake 13 to 14 min. or until sides are firm but centers are soft. Let stand 1 min. Carefully run small knife around cakes to loosen. Invert cakes onto dessert dishes. Serve immediately each topped with 1 Tbsp. of whipped topping.