Japanese Buttercream (バタークリーム) is pretty much like European buttercream.  Soft butter is mixed into a sweet meringue with flavoring such as vanilla or liqueur.  Buttercream cakes used to be very popular in Japan. Ever since whipped heavy cream cakes emerged in the 1970s, though, buttercream cakes were eclipsed and have become next to nonexistent today.  That may be because Japanese people tend to like lighter and less sweet cakes, and they think buttercream cakes are overly sweet and heavy.