1. Preheat the oven to 350 F. Lightly grease 6 6-ounce ramekins and place these in a roasting pan.
2. Spread the cubed croissant pieces onto a parchment-lined baking tray and toast for 10 to 15 minutes, stirring once halfway through toasting, then let them cool to room temperature.
3. Bring the whipping cream and milk up to a simmer and pour this over the chopped chocolate. Let it sit for a minute, then whisk until the chocolate has fully melted.
4. In a separate large bowl, whisk the egg, yolks, sugar and vanilla together. Slowly pour the hot cream into the egg mixture while whisking, then whisk in the Irish Cream. Stir in the croissant pieces to fully coat them, then let this mixture sit for 10 minutes.
5. Stir in the milk and chocolate chips, if using, then spoon the pudding mixture into the prepared ramekins. Pour boiling water into the roasting pan so that it comes halfway up the sides of the ramekins. Bake for 25 to 30 minutes, until the pudding springs back when gently pressed. Carefully remove the ramekins from the roasting pan to cool for 10 minutes, then invert onto a serving plate and serve. Recommended to serve with a scoop of ice cream on top.
6. You can prepare and bake these puddings up to a day in advance and chill until ready to serve. Ideally they are best served warmed and most easily reheated in the microwave.