Makes 15 portions
Melt chocolate and butter together until just barely melted. Remove from heat and cool slightly.
Beat 1/2 of the sugar with the egg yolks until mixture is light and creamy and doubled in volume.
Beat egg whites to soft peaks, while still beating, gradually add remaining sugar and beat until stiff peaks form.
Whip cream to stiff peaks.
Fold chocolate/butter mixture into yolks, and then fold in whipped cream, rum and vanilla.
Gently fold in whites to retain as much volume as possible. Chill to set.