I have been asked to do a pillow cake, but they want it covered in buttercream and not fondant. Has anyone made one this way?? How do you get it smooth on the bottom??

With a lot of work! lol - I personally wouldn't want to do it, but I would bet it is possible. You would just have to spend a lot of time smoothing and getting it perfect. I personally would talk them into fondant and just tell them there will be plenty of BC underneath to eat!