exactly. totally sympathise, but this is a circular thing. she knows that in the end she will get a smack so she goes for it until she does.
what about right at the beginning of the tantrum, having empathised and having told her how it makes you feel when she does this (also worth talking to her about that when she is not actually doing it) you say that you'll count to five and then do something that will really hurt in a longer term way, such as taking away a prized toy for 24 hours. It won't work first time but if you follow through i'm sure it will within quite a short space of time.
do you praise her a lot when she isn't being difficult, and notice when she is reasonable and well behaved, giving treats without announcing them in advance if she has been good company one afternoon for example? that helps to ime.
also if smacking is so effective, why wait for 2 hours, why not do it straightaway? i imagine that that would stop working very quickly though. part of it is that she has worn herself out and smacking is the cue to stop maybe.