The ideal way to serve a magnum ice cream is to let it almost totally defrost, then re-freeze it a bit so that when served the ice cream on the inside is almost like a liquid. This means you can break the chocolate on the outside and suck the nice liquid ice cream out, excellent news.
Magnum's can be too cold. This has been proved by scientists. Probably. They did a study on it in Texas or something, I reckon.