Swiss meringue buttercream is a frosting made by whipping softened butter into a cooked Swiss meringue (egg whites and sugar heated together over a double boiler, then whipped to stiff peaks), producing a smooth, silky, spreadable icing.
To make it, egg whites and sugar are heated to about 160°F/71°C to dissolve the sugar and pasteurize the whites, whipped to stiff peaks, cooled, and then butter is beaten in gradually until light and fluffy.
Compared with American buttercream (butter beaten with powdered sugar), it's noticeably less sweet, with a lighter, airier texture and a more stable structure that holds up better at room temperature, making it a common choice for wedding cakes and other cakes meant to sit out. Flavor variations include chocolate, coffee, and fruit purees.