Heat oven to 425°F.
Butter 4 small custard cups; place on baking sheet.
Microwave chocolate and butter in medium microwaveable bowl on HIGH 1 min. or until butter is melted; whisk until chocolate is completely melted. Stir in sugar. Add whole eggs and egg yolks; mix well. Stir in flour. Spoon into prepared cups.
Bake 13 to 14 min. or until edges of desserts are firm but centers are still soft. Let stand 1 min. Carefully run knife around cakes to loosen; invert into dessert plates. Serve warm with COOL WHIP.