Melt semi-sweet chocolate and butter in double boiler. Set aside and keep warm.
Beat egg yolks and 1/3 cup sugar with electric mixer until thick and ribbony.
Stir yolk mixture into warm chocolate mixture.
Beat egg whites, adding remaining 1/3 cup sugar a little at a time, until stiff and creamy. Fold egg white mixture into chocolate mixture.
Get 6 heatproof coffee cups or mugs (or use ramekins). Butter and sugar insides of each.
Fill each 3/4 full with chocolate mixture. Place 1 piece of bittersweet chocolate into the center of each filled cup.
Bake at 350° F. until souffles are puffed and just beginning to crack (should be about 15-20 minutes).
Remove from oven and cool slightly. Serve with whipped cream on top.