Cut the croissants in half lengthwise. If I they are still soft inside, put them on a plate and let them dry out a bit more (overnight if necessary).
In a bowl, whisk together the egg, sugar, milk, vanilla and cinnamon (if used).
Dunk each croissant half into the mixture so that it's fully coated. Set them aside on a plate.
Heat a large non-stick frying pan over low heat, then melt the butter.
Add the croissant halves, cut side facing downwards, then increase the heat slightly to medium.
After 3 minutes or so they should be golden brown, so flip them over and cook for another 2 minutes or so on the other side.
Remove from the pan and serve the a top and bottom pieces with your chosen toppings and sauces.