The taste in PL seems to be for bars of filled chocolate. This sort of thing:
Very popular, but the best of the best, for me, is something from the Solidarność chocolate people (go on, get your own back pp). Chocolate-covered plums. You can get them in very attractive presentation tins, and shops sell them loose for about 23zł/kilo. They are fabulous. There are imitators who market inferior products, so beware. Accept nothing but Solidarność chocolate plums.