The process. Sorry, but I can't keep the terminolgy straight.
From egullet:
Italian meringue buttercream:
a meringue that is whipped and sweetened with a cooked sugar syrup (the syrup is usually cooked to a soft ball stage), then butter is added.
Swiss meringue buttercream:
the egg whites and sugar are heated together, usually in a water bath, until very hot. Once hot, you whip the sugar/whites mixture til you get stiff peaks, then add butter.
IMBC, because of the extremely hot sugar syrup added to it, is usually more stable.
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