A Polish Butterfly is best made with 90% Polish Grain. Because of this it can be very very hard to make a 'real' Polish Butterfly outside of a few countries such as Israel and Poland where grain alcohol is readily available.
When you drink one of these you are downing 50 ml of what should be 80% alcohol. This goes a long way to exaplaining its leathality.
Depending on where you are it can be served in several different ways. The prime difference is whether or not it's been mixed, as oppossed to the ingredients just being poured into each other. When served pre-mixed and chilled it seems to have an increased potency, although that may be purely psyhcological.
If you ever want a Polish Butterfly in Jerusalem then try a bar called "The Tavern", anyone in the center of town should be able to tell you where it is.