One of my many imaginary friends just asked me why buttercreams made with vegetable shortening leave that greasy sensation in your mouth when you swallow them. “Manny,” I said, “It’s all about melting points.”
The reason real buttercreams are so silky smooth is because they’re made with real butter, and butter has a melting point just below body temperature (somewhere between 90 and 95 degrees depending on the butter). Put it in your mouth where it’s a steamy 98 degrees or so and it melts, creating that rich delectable sensation. The faster it melts the “finer” the sensation. Fast-melting solids are said to have a “sharp” melting curve.
There are of course other fats that share some of these properties. Cocoa butter is one, which is why chocolate has such a creamy mouthfeel (that’s hipster industry speak for “the sensation it creates in your mouth”). Like butters, chocolates have different melting points and curves, which is what the science of candy making is all about, and why, say, the firmer exterior of an expensive truffle slowly dissolves in your mouth while the interior liquifies.
A fat that doesn’t quite share these properties is shortening. Unlike butter and cocoa butter, its melting point is above body temperature, somewhere right around 108 degrees. Which means that unless you’re delerious with Dengue Fever (in which case, let’s be honest, who gives a damn about chocolate?) any truffle made from shortening will end up tasting like chocolate-flavored lard.
A similar thing of course happens with a so-called “American” buttercream. The butter that’s in it (and better American buttercreams have a higher proportion of real butter) melts, giving you a silky sensation initially. But the shortening that’s present will stay unmelted, leaving a slick, greasy-feeling film.