In place of the milk so that the cakes won't have to be refrigerated overnight??
AI use non dairy creamer (like Hood Country Creamer) and do not refrigerate, and my frosting stays just fine at room temp, assuming it's not a super hot day. I use vegetable shortening, margarine and powdered sugar plus a flavoring ingredient like clear vanilla. I've tried using water or the powdered creamers and it just does not work as well for me. I find it gets too crusty and does not stick to the cake, almost like it peels away. Good luck!