1. In the bowl of an electric mixer fitted with the paddle attachment, beat butter until soft and fluffy. Add vanilla and mix to combine.

2. In a second bowl of an electric mixer, mix together egg whites and sugar. Set bowl over (but not touching) a saucepan of simmering water; whisk constantly until sugar has dissolved, 3 to 5 minutes. Transfer bowl to a mixer fitted with the whisk attachment and beat on high until stiff, glossy peaks have formed, about 4 minutes.

3. Reduce speed to low and add butter mixture, 1/4 cup at a time, mixing well after each addition. Continue mixing until smooth. Use immediately or transfer buttercream to an airtight container and refrigerate for up to 1 week. Bring buttercream to room temperature and remix before using.