Microwave chocolate and butter in large microwaveable bowl on MEDIUM (50%) 2 min. or until butter is melted. Stir with whisk until chocolate is completely melted.
Add 1-1/2 cups sugar and flour; mix well.
Add whole eggs and egg yolks; beat until blended.
Pour into prepared cups.
Bake 14 to 15 min. or until cakes are firm around the edges but still soft in the centers.
Let stand 1 min. Run small knife around cakes to loosen; carefully unmold onto dessert plates.