4 large egg whites
1 cup sugar
1/2 teaspoon salt
1/4 teaspoon cream of tartar
2 cups (4 sticks) unsalted butter, at room temperature, cut into 1/2-inch pieces
1 teaspoon pure vanilla extract
In the top of a double boiler set over simmering water, whisk together the egg whites, sugar, salt, and cream of tartar. Continue whisking until the mixture becomes too hot to touch comfortably, 7 to 10 minutes. Transfer the mixture to the bowl of an electric mixer fitted with a whisk attachment. Beat on medium-high speed until the egg whites hold a stiff peak, about 8 minutes. Add the butter, one or two pieces at a time, beating after each addition until incorporated. Beat in the vanilla. Use at room temperature.