Regarding the hot caramel icing ... is the icing put on a warm cake or does the cake need to be cooled first?

Thank you ... this was the correct recipe. I'm new here and didn't realize that I needed to include the recipe. I compromised and put the icing on a barely warm cake. I think I cooked the icing too long - it is like hard fudge on the cake. Oh well ... it will get eaten. Thanks for the response.