This recipe calls for olive oil because it doesn't thicken when chilled, so it makes the batter runnier and easier to pour into the pan. (The taste is not noticeable, but you can use another mild-flavored oil, too.) If you prefer to use melted butter or coconut oil, instead, you might want to make sure your eggs aren't cold from the fridge, or the batter might thicken when you mix it.