as a rule, the cheaper the icecream, the less likely it is to come off.
I do the following: wet the stain with warm water, pout some fairy liquid on it and rub it on, rub some vanish sopa on top of that and put it in the normal wash. It works everytime, apart from when MIL gives DD icecream (no idea where she buys it from but I'm not too keen on it).