- 6 sticks unsalted butter
- 2 Tbs. vanilla extract
- 2 1/2 cups sugar
- 6 egg whites
- 1 3/4 tsp. salt
- 1/4 tsp. cream of tartar
In the bowl of an electric mixer fitted with the flat beater, beat the butter on medium speed until creamy and smooth, about 2 minutes. Beat in the vanilla until combined, about 1 minute. Transfer to a bowl and set aside. Thoroughly wash and dry the mixer bowl.
In the clean mixer bowl, using a handheld whisk, whisk together the sugar, egg whites, salt and cream of tartar. Set the bowl over but not touching simmering water in a saucepan and whisk constantly until the mixture registers 140°F on a candy thermometer, about 10 minutes.
To test if the sugar has completely dissolved, rub a small amount of the egg white mixture between 2 fingers; it should be smooth and not grainy. If it feels grainy, continue to heat and retest every 30 seconds until smooth.
Remove the bowl from over the water. Fit the mixer with the whisk attachment, set the bowl on the mixer and beat on high speed until the mixture is cool and soft peaks form, 7 to 10 minutes. Gradually add the butter about 1 Tbs. at a time, beating well after each addition until smooth. Refrigerate the buttercream for 15 minutes before using. Makes 6 cups.