Melt the butter, blend in the flour and sugar, and mix well. Add the cream. Cook, stirring, over medium heat until thick. Beat a small amount of the hot mixture into the egg yolks. Return to the saucepan, blend well and cook 1 minute longer. Cool slightly.
Stir in the chocolate which has been melted over hot water. Add the vanilla. Beat the egg whites until stiff but not dry. Fold into the chocolate mixture.