Like Cat said it will keep just fine at room temperature. I make several batches of this icing every week and it will keep fine at room temperature for up to two weeks. I never refrigerate my icing or my cakes.
I also use it to make my chocolate buttercream just like Cat said. Using UNsweetened chocolate squares. You also have to add extra milk (I use half and half) when adding the chocolate because it thickens the buttercream. I add only a teaspoon at a time so that I'm careful not to add too much liquid. This makes the absolute BEST chocolate buttercream that I've ever had
Not only does it taste wonderful but it still crust and smooths the same as the "original" icing, which is a huge plus for me. I love this recipe.