- 1. Place the egg whites and sugar in a metal bowl and set over a saucepan filled with 2 inches (5 cm) of gently simmering water. Whisk constantly (but not vigorously) until the mixture reaches 150°F (65°C) on a candy thermometer, about 6 minutes.
- Use electric beaters or transfer the mixture to the bowl of a stand mixer fitted with the whip attachment and whip at high speed until the meringue has cooled to room temperature (it will hold a stiff peak by then).
- With the mixer running at high speed, add the butter a few pieces at a time. At first the meringue will hold its volume, then the buttercream will deflate a little and become very creamy yet fluffy looking. Beat in the vanilla. Use immediately, or cover and chill or freeze for later use.
- To use the frosting once it’s been chilled, let it come to room temperature on the counter. Rewhip the buttercream to make it fluffy and spreadable.