Preheat the oven to 190C/375F/Gas 5.
Melt half the butter and use it to grease a 2 litre/3½ pint soufflé dish.
Put the Christmas pudding into a food processor and pulse to break it all up. Leave it in the food processor.
Bring the milk to the boil in a small saucepan. Add to the Christmas pudding, blending it until it is well combined. Add the yolks to the mixture one at a time, blending between each addition.
Beat the egg whites with a pinch of salt in a bowl until stiff peaks form when the whisk is removed from the bowl.
Fold the beaten egg whites carefully into the Christmas pudding mixture.
Spoon the mixture into the dish and bake for 40 minutes. Take care not to burn the top.
Remove from the oven, dust with icing sugar and serve with brandy butter ice cream.