It is a silky smooth, super creamy, and delicious frosting. It’s made by whisking egg whites and sugar over a pot of boiling water. It’s then whipped until they the mixture reaches stiff peaks. That creates the Swiss meringue! Adding unsalted butter to the Swiss meringue, a tablespoon at a time, turns the Swiss meringue into a buttercream frosting! It’s a popular frosting because it isn’t as sweet as American buttercream.