Place the egg whites and sugar in the metal bowl of the stand mixer. Put the bowl on top of a pot with boiling water, not touching the water. Whisk constantly until the sugar dissolves and not grainy anymore.

Place the bowl on the stand mixer and whip on high until it forms a stiff peak. About 10 minutes.

With the mixer on low, beat in the butter. After all the butter is added, increase the speed to medium high and mix until smooth.

This buttercream can be stored on the counter for a few days or in the refrigerator. If store in refrigerator, bring it to room temperature before using.