What is "Buttered Sugar"?
A search of the internet for "buttered sugar" comes up exclusively with variations of the this list on various websites that list alternative names for sugar. 
Of these only one "Returning to nature" lists it as a synonymy for buttercream. 

It is possible that site is correct and "buttered sugar" is a possibly rarely used alternate name for buttercream. I would suspect that it is more likely that there has been an error at some point in the past that has been copied between these sites.