This recipe is from Dorie Greenspan and I adapted it a bit. This frosting is a little complicated to make, but the taste is worth it.
Italian Buttercream Frosting
4 room temperature egg whites
1 c. of sugar
3 sticks of butter, room temperature
1 t. vanilla
Put the egg whites and sugar in a double boiler, or if you don't have one, use a mixing bowl or small sauce pan set in a pan of simmering water. Using a hand mixer, beat the eggs and sugar (while in the double boiler). Beat it for 4 minutes. Remove from the stove and beat for five minutes, until it has cooled down. Continue beating, adding the butter one pat at a time. Add in the vanilla. Then beat for another 10 minutes.