I make buttercreams quite a bit--with vanilla, with rum, and as a "crème brûlée" (with even more vanilla and some caramel crunch). I use @Kerry Beal's recipe. It is based on fondant and butter plus white chocolate to make it firm up (and add yet more vanilla). Since it's her recipe, I don't feel I can post it, but she will probably notice this and respond. Contrary to expectations, the fondant is not overwhelmingly, cloyingly sweet in this recipe. I tried Jack Daniels and also a "boutique" bourbon as flavorings once, but surprisingly to me, the booze flavor was lost--it would take so much of it to taste that the buttercream would be practically liquid. Dark rum, however, is delicious. These are quite popular--it's that old-timey thing, I guess.
Why don't you want a butter ganache? That would also work with what (I think) is the right texture. It's quite similar to the fondant approach.
