I am working with the powdered sugar, corn syrup, vanilla and milk. My question is, with having the milk ingredient does the cookie have to be refrigerated after it is decorated? I am concerned about leaving it out. In doing so, I know my mom used to do the powd sugar, butter, vanilla and a little bit of milk frosting and we never refrigerated our cookies...any help on this is appreciated.

I know that you can substitute water fro the milk without any problem. That's what I do when I use a glaze icing.

The high sugar content acts as a preservative. I use glaze and rarely ever refrigerate the cookies. The glaze does not like getting cold while it is setting, so I would highly recommend no refrigeration for at least 24-36 hours after icing the cookies. Leftover icing will stay fresh in the refrigerator for 2-3 weeks.