Swiss meringue buttercream is the gold standard for high-end cakes for a reason: it’s stable, silky, and significantly less sweet than its American counterpart. While the process of whisking egg whites over a double boiler can seem daunting, the result is a frosting that pipes like a dream and withstands warmer temperatures.
Temperature is Everything
The most common failure point is the temperature of the butter. It must be slightly cool to the touch—not room temperature and certainly not melted. If your meringue is too hot when you add the butter, you'll end up with soup. If it's too cold, you'll get curdled chunks.
The fix? Patience and a good thermometer. Whisk your meringue until the bowl is completely cool to the touch before adding a single cube of butter.
