DO NOT OVERBEAT THE EGG WHITES! Whip until they just reach the stiff peak stage- if overbeaten, they will get dry and won't hold as much air. To save overbeaten egg whites, add another egg white and beat.
It's best to use a stand-alone electric mixer, so that the sugar syrup can be poured in between the bowl and the beater and be evenly incorporated.
The egg white and sugar mixture can continue to be beaten until cool (room temperature) without worrying about overbeating the egg whites.
If the frosting looks like cottage cheese for a while, keep beating and adding butter. It usually looks like that for a while.
If the frosting is soupy once all the butter has been added, put it in the refrigerator until cooled and whip again.
Instead of vanilla extract, other flavorings can be added. For example, orange extract and orange zest can be added to make orange buttercream.
Refrigerating the buttercream is not necessary, but recommended. It keeps for approximately four days.
Bring the buttercream to room temperature before rewhipping or it will separate. If it does separate, just continue beating, it will come back together after around five minutes.