I've made and like flour frosting (ie thicken milk with flour, add flavoring, sugar etc) cool and add to butter - beat etc.

But i'm not sure how long it can stay unrefrigerated since it has milk in it? thus wondering if I can substitute water for the milk?  any ideas?

Here is the Recipe:

1/4 cup flour
1 cup sugar
1 cup milk
1 cup butter cut in cubes
1 tsp vanilla

Heat milk, wisk in flour and stir for 4-5 minutes until thick. 

Beat sugar with butter. once milk mixture is cool slowly add to butter mixture and beat. add flavoring.

I don't think it will taste the same, the milk gives it a richer taste. I make a similar frosting but it has cornstarch instead of flour. And it does keeps well at room temperature for a few days.