HEAT oven to 425°F.
BUTTER 4 small custard cups; place on baking sheet.
MICROWAVE chocolate and butter in large 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 sides of desserts are firm but centers are still soft. Let stand 1 min. Carefully run small spatula or knife around cakes to loosen; invert into dessert plates. Serve warm with vanilla ice cream.