I never had the cupcakes you mention, but "italian meringue" buttercream is extremely smooth and soft and creamy. It's also very easy, actually, if you have a stand mixer.
It's not an italian buttercream, though, and cakes, as we know them, butter cakes, are unknown here and those that they do make are usually some sort of genoise (if made in a bakery - home cakes are really dry) and rarely have buttercream in the way we think of it. But anyway, irrelevant.
I kind of would be surprised if a big chain like that would be making italian meringue buttercream, and suspect there is some sort of concoction involving emulsionates and all that, in their frosting and certainly shortening (crisco) rather than butter. I also remember years ago someone making a frosting based on a flour/milk/sugar roux (i'm trying to remember), then beating in shortening (it's white, otherwise it will be yellowish, but of course, much better). This is very fluffy and smooth (the sugar is cooked so there is no granularity, if i remember it right). Anyway, if the frosting is white, it;s not butter, but why does it have to be white anyway? |