Yes, I know, it's a butterfly, not a rosette...
The irons come in many shapes, some undoubtedly in the shape of a rosette.
I have been making rosettes for a very long time. I use my mother's recipe, as published the Christ Lutheran Women's cookbook. As you can tell, it is a well-used reference. There are many Scandinavian recipes in it, and most of the rest come from descendants of Scandinavian immigrants that settled in the northern plains.
Making rosettes can be kind of tricky. I have some other irons (I think) somewhere, but don't use them. They are not seasoned. I tried them once, but the batter stuck horribly. Sometimes the first one or two stick, but once you get going, they don't stick, come off the iron easily with the gentle aid of a fork. One does need to wear comfortable shoes, though, as you are standing at the pot of oil, making them one at a time. Someone else needs to answer the doorbell and telephone.
This is one of our family's traditional holiday sweets. I dust them with powdered sugar.