Beat egg yolks; beat in sugar and flour. Place in large sreving bowl. Scald milk and pour slowly over mixture. Stir in the rum. Serve with zwieback.
If sago is used instead of flour, boil milk and slowly stir in sago and a pinch of salt. Cook slowly about 20 minutes, until sago is clear, before pouring over the egg, sugar and flour mixture and adding the rum.