For the icing, combine the icing sugar with 2 - 3 tablespoons for but milk (add a tablespoon at a time) until the mixture turns into ever so slightly runny consistency. (fitfoodienutter.com)
It has a thin runny consistency that blends into the skin easily. (facetocurls.com)
So you know it's a sweet milk with thicker than heavy cream but still runny consistency. (bake-no-fake.com)