Use to make Cake Power founder Kate Sullivan's Puppy Cupcakes.
In the bowl of an electric mixer fitted with the paddle attachment, beat butter until soft and fluffy. Add vanilla and mix to combine.
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.
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.