Use this brown sugar buttercream on our Triple-Layer Apple Cake. If the mixture seems to have separated after all the butter is incorporated, beat on medium-high until smooth. Reduce speed to low; beat 3 minutes more.
In a heatproof bowl set over (not in) a pan of simmering water, whisk together egg whites, sugar, and salt. Cook, whisking constantly, until the sugar has dissolved and the mixture is warm to the touch.
Transfer to the clean bowl of an electric mixer. Beat on medium speed until fluffy and cooled, about 15 minutes.
Raise speed to high; beat until stiff peaks form. Reduce speed to medium-low; add butter, 2 to 3 tablespoons at a time, until fully incorporated.