4 large eggs
1 cup sugar
2 teaspoons vanilla extract
1 pound (4 sticks) unsalted butter, softened, each stick cut into quarters
1. Combine eggs, sugar, vanilla and salt in bowl of standing mixer; place bowl over pan of simmering water. Whisking gently but constantly, heat mixture until it is thin and foamy and registers 160 degrees on instant read thermometer.
2. Beat egg mixture on medium-high speed with whisk attachment until light and airy, and cooled to room temperature, about 5 minutes. Reduce speed to medium and add butter, one piece at a time. After adding half the butter, buttercream may look curdled; it will smooth out with additional butter. Once all butter is added, increase speed to high and beat 1 minute until light, fluffy and thoroughly combined. Can be covered and refrigerated up to 5 days.
From Americas Test Kitchen.