Make sure the bottom of the mixer bowl does not touch simmering water. Your egg whites may become a sweet scrambled breakfast dish! Finished buttercream may be kept covered and refrigerated for a week. Professional chefs prefer to use pasteurized egg whites - no need to separate fresh eggs, no wasted yolks and safer all-around. Buttercream also freezes well for up to 6 months. Chilled buttercream must be brought to room temperature before beating, otherwise it will curdle. If such a disaster strikes, do not panic! Keep beating over simmering water and eventually the mixture will come together again.