Place chocolate and butter in a large bowl and microwave on high for about 1 minute, until chocolate is almost melted. Stir with a wire whisk until chocolate is completely melted.

Whisk in sugar until well blended.

Whisk in eggs and egg yolks. Stir in flour.

Spray 4 (3/4 cup) custard or souffle cups with non-stick spray. Divide batter between baking cups.

Place baking cups on a baking sheet, and bake at 425 degrees F. for 13-14 minutes, until sides are firm but centers are still soft.

Remove from oven and let stand for 1 minute. Run a knife around sides to loosen. Invert cups onto dessert plates to remove cakes. Top with a dollop of whipped cream if desired, and serve immediately.