Charming to see an old tradition brought back. In my family we use applesauce
So we put half of the batter in, let it set up for a few seconds then drop a small amount of applesauce on top then the rest of the batter so the applesauce is a tasty surprise on the first bite when it comes out of the pan.
You can also make Ebelskiver on a griddle- like a small pancake but the taste is the same.