I have found a few MM fondant recipes and a couple said to add 1/2 cup butter. Anyone know which one works better? Butter or no butter? I have to make an army tank cake this weekend and don't want to screw up the fondant.

Most home-made fondant recipes have shortening, not butter in it.. (even marshmallow fondant) I would also make it a day before you want to roll it out. For classic fondant, I use The Cake Bible recipe and for Marshmallow fondant I use Wiltons. You can also buy a tub of it at Micheals.