Hi Sarah,
I've never added anything to them. I just eat them with light butter. :) I'm not sure what makes them the way they are. I think it has something to do with the liquid and heat making steam to help them rise, then the egg helps them stretch and hold together so you get a crusty outside and kind of airy hollow inside. I could be wrong since I'm not really a food scientist. I use to have these little clay pots that were made for making popovers. They were so cute. :)