Our son like his pain au chocolate with butter spread on it.
I don't see an issue with that. So, I have always allowed it. But, my wife disagrees categorically based on the premise that a sweet pastry does not need butter (she butters her croissant). She says it is a cultural difference point (she is British and I am Middle Eastern). We each think the other is unreasonable about this.
Thought we would post here for an idea what most people think.