It's a bit obscure, though. (That should make it more interesting.)
Not all Dutch people know about this, so it might be a regional custom to bake (or actually fry) these small, hollow waffles on New Year's Eve; I don't know which region, though.
I was taught how to do these by my family. My grandmother always made them because she preferred them to
oliebollen which are quite massive and can lay on the stomach like the proverbial brick. Both my mother and her sister have their own set of irons, as does my father. And of course, so do I.
The irons can be hard to find, but older houseware shops in the smaller villages may have them. Alternatively, I've been told some of the more upmarket cookery stores now stock them because it's quaint and interesting to have obscure cooking utensils nowadays.
Without further ado, here's a picture of my 'vlindertjes-ijzers' (butterfly-irons) and the Vlindertjes I baked on New Year's Eve.

As you see, they don't all look like like butterflies. There are wheels and stars and I've seen pictures of card symbols too. But in my family they're referred to as Vlindertjes, no matter which shape they are.
These waffles are made from a normal waffle batter, liquid enough so you can dip the iron into it. The iron should be hot enough to sizzle ever so slightly when you dip it into the batter, so be sure to heat it up in the deep fryer first. Don't dip it in all the way or you'll never get the batter off! The top of the iron should remain free from batter.
If the iron is hot enough the waffle will come loose from it while it's being fried. You may need to help a little with a fork, though. Deep fry it in hot oil until a nice golden brown, turn it over so the hollow side is at the bottom and lift it from the oil. Put it on some absorbent paper to get rid of excess oil. After they've cooled down, the Vlindertjes will be nice and crispy. Sprinkle with a generous amount of confectioner's sugar, and enjoy.
Oh, and it's perfectly normal if the first three waffles stick to the iron. In fact, there traditionally should be a bit of a struggle before you get it right. Or so I tell myself!