I was asked to do a last minute cake as a favor. The night I need to complete the cake I have another event that I must to go to first. In order to not be up forever that evening, I was hoping to make my buttercream (with water) the evening before. How would you go about storing it? Can it sit out or does it need to be in the refrigerator.

If I am using the butter cream within the week, I leave it out on the counter, well wrapped. If I am storing it for a week or 2 I use the refrigerator. For very long term storage, I will freeze it for several months (make sure you label it if you have thinned it out - you won't remember if you don't).

BC with butter in fridge 2 weeks.
BC with shortening/water counter 2 weeks
BC with shortening/milk fridge 1 week
All wrapped tight. They have all worked well with no problems.