Grease a 1.2 litre (2 pint) shallow, round, ovenproof pie dish. Slice the croissants thickly and spread the butter over one side of each cut face of croissant. Stand the croissant slices upright and close together in the dish to completely fill it.
Mix the sugar and spice together, then spoon over the croissants and between the gaps. Stand the dish in a large roasting tin.
Beat the milk, eggs and vanilla essence together, then strain into the dish. Leave to stand for 15 minutes.
Pour hot water from the tap into the roasting tin to come halfway up the sides of the pie dish. Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for about 25 minutes until the pudding is golden and the custard just set.
Lift the dish out of the roasting tin, dust with sifted icing sugar and serve the pudding warm with a little pouring cream.