2cupsunsalted butter, at room temperature (1 pound, 450 grams)
1teaspoonvanilla extract
Instructions
Combine the water and sugar in a saucepan over medium heat. Bring the mixture to a boil, stirring occasionally. Once it boils, stop stirring and insert a candy thermometer.
Meanwhile, whip the egg whites in the bowl of a stand mixture to soft peaks. When the sugar is at 238°F, add it to the egg whites in a slow steady stream with the mixer on medium speed.
Increase the speed to high and beat until cooled to room temperature.
Add the butter a few tablespoons at a time, while continuing to mix on medium high speed. Add the vanilla. Increase the speed to high and beat until smooth.