These are very similar to Aebleskivers. I have the Lodge cast iron Aebleskiver pan and it will work nicely for these too.
Easy to prepare but, admittedly, some cooks are intimidated with the technique necessary to make them turn out just right. It takes a bit of practice but the end result is worth the effort. Some turn them with a fork; I use stainless steel knitting needles to turn them.
That said, you don't actually need a special pan to make poffertjes. A common fry pan of good quality will work fine as long as you keep them small. It's just that the Poffertjes or Aebleskiver pan produces a clever little package that adds to their interest on the plate.