Brown Sugar Buttercream
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.Advertisement
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.