In a large glass bowl, break up bittersweet chocolate. Add butter, and microwave on high for 2 minutes until melted and smooth, stirring halfway through. Stir until smooth; cool slightly.
In a medium-size bowl, whip eggs and sugar on medium-high speed of an electric mixer or by hand, until tripled in volume, about 6 to 7 minutes.
Fold egg mixture into chocolate and divide among 8 well-buttered 6-ounce ramekins or molds. Bake at 350°F for 15 to 17 minutes or until top is just set.
Invert cakes onto plates and serve with mint ice cream and fresh mint, if desired.