Melt butter and chocolate in the microwave stirring every 15 seconds until smooth.
Whip eggs, yolks, sugar and vanilla with a mixer for about 10 minutes on high speed.
Fold melted chocolate and butter into the egg mixture. Fold in flour until just combined. Grease four 6-oz. ramekins, then spoon the mixture into the ramekins.
Place one dark chocolate square in each ramekin.
Bake at 450° F for 10-15 minutes. The center will be quite soft, but the top and sides will be set. Let sit out of the oven for about 5 minutes, then unmold onto a plate or serve in ramekins with a scoop of vanilla ice cream.
* These are best served warm right out of the oven, but leftovers can be reheated in the microwave.