Can anyone tell me if buttercream made with milk and butter can be left out over night?

Did you make an American buttercream: lots of powdered sugar, fat (shortening, margarine, butter or any combination) and a little milk or cream?

Most American b/c's are shelf stable, even when milk or cream are used as the liquid.

Sugar is hygroscopic, or moisture absorbing.
The powdered sugar in American b/c's controls the water activity in the butter/margarine and/or milk or cream in the mixture. (Butter and margarine are both 80% fat and 20% water, while shortening is 100% fat.)

Water activity & microbial growth:
(Prolonging Bakery Product Life.)

http://tinyurl.com/6fbkcz

WJ Scott in 1953 first established that it was water activity, not water content that correlated with bacterial growth:

http://tinyurl.com/bmsato

Formulating for increased shelf life:
(Decreasing water activity results in hostile environment for bacteria.)

http://tinyurl.com/csu2b9

HTH

it is 1/2 cup veg short.
1/2c butter
vanilla
confectioners sugar
2 tbsp milk

I think what I was told by a wilton instructor is yes you can but once you cut into it that it has to be refrigerated. I could be wrong, you may have to refrigerate at all times.

It will be fine I always leave mine out and it contians all butter!

You should be fine!

Hi and Welcome on your 1st post, iwantalicakes.

CC is a great place to exchange ideas and learn new techniques, etc. But CC is also a great place to get updated info.

Frostings, and fillings that are perishable and completed cake/s with perishable frostings/fillings do require constant refrigeration, except when being served (2 hr. limit of "out" time).

However, frosting and fillings that are shelf stable and completed cake/s with shelf stable frostings/fillings do NOT require refrigeration.

As a matter of fact, refrigeration tends to stale baked goods, especially bread very quickly.

Science of staling caused by refrigeration:

http://www.patentstorm.us/patents/6635289/description.html

http://www.ochef.com/894.htm

HTH

thank you everyone!