Quoting cookie monster (a and p):" Is the butter cream frosting okay to leave out overnight after covered in fondant? I know the fondant ... [snip!] ... I know the fondant will be because there isn't any perishables in the fondant just confectioners sugar and marshmallows."
yep. every decorator I know finishes their cakes the night before and leaves them out. as long as it's not any warmer than normal room temperaure. I wouldnt leave it out on a back porch in july for an entire day before serving haha. if you have a cool room to leave it in that's best
but yeah. the high sugar content will act like a preservative.
if you're uncomfortable with it, just use water instead of milk in the recipe.
if you put a fondant decorated cake in the fridge everything is going to get sticky and soft and colours run from the moisture