Put the milk, cream and cinnamon in a pan and heat gently until just comes to the boil.
In a large bowl, beat the eggs, caster sugar and vanilla until creamy.
Pour the warm milk gradually over the egg mixture, while stirring constantly. When you get a smooth, thick cream add the Baileys and set aside.
Grease a baking dish with butter.
Cut the croissants into long 3cm thick slices, then spread some butter on them.
Arrange the croissants in the baking dish, letting them overlap each other.
Scatter the chocolate bits or dried fruits on top, tucking them between the slices (if used). Pour the custard over the croissants.
Let it rest for 15 mins, while you preheat the oven to 180°C.
You can sprinkle some sugar over the croissants, if you want a crunchy, caramelised crust, then bake for 35-40 mins in the middle of the oven.
Whet the top is golden and the custard has just set (it should wobble a little), then its’ ready to serve.