I Am in the middle of making buttercreme frosting and just realized tht I am out of milk!! Can I use half and half and just add water. I need 1 cup of milk. anyone know the proportions?

I have used regular cream in buttercream before and it has worked fine. Therefore I would suggest using the half and half as is, ie. no water.

As for proportions, just add a little at a time until the buttercream is the consistency you want.

I hope that helps or that someone with more experience responds.

Thanks, I can try that, but will it have to be refriderated, or can it just be treated the same as if make with milk?

you can treat it the same

I know this is after the fact but just for your info you can use *any* liquid in icing recipes.
Some opeople have used fruit juice, yogurt, even water w/o problems.
You do not need to refrigerate your cake w/icing using dairy as the sugar perserves it.
I would frig any left over icing unless you are going to use it the next day.