Swiss meringue buttercream is made by heating egg whites and sugar to 160°F before whipping, creating a stable, silky buttercream. Unlike American buttercream, it's less sweet and more sophisticated. Heat the whites and sugar in the mixer bowl over simmering water, whisking constantly. Once it reaches temperature, transfer to the mixer and whip until cool and glossy. Gradually add room-temperature butter. The mixture may look curdled initially - keep beating and it will come together into smooth, pipeable buttercream.