Source: The Martha Stewart Show, April 2011
- 2 cups (4 sticks) unsalted butter, room temperature
- 1 1/2 tablespoons pure vanilla extract
- 5 large egg whites
- 1 1/2 cups granulated sugar
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.