Place chocolate in heatproof bowl, place over pan of simmering water, stir chocolate until melted, remove from heat. Add butter, stir until melted, stir in egg yolks, 1 at a time; transfer mixture to large bowl, cover, cool.
Beat egg whites in small bowl with electric mixer until soft peaks form. Fold the cream and eff whites into chocolate mixture in 2 batches. Pour mixture into 6 serving dishes (2/3 cup capacity), refrigerate several hours or overnight. Serve with extra whipped cream and chocolate curls.