Put the milk, sugar, vanilla and rice into a large saucepan over a medium heat and bring to the boil. Turn down to a gentle simmer and cook for 30 min, stirring from time to time. During the last 5 min, stir slowly and continuously to prevent the rice from sticking. At this stage it will be three-quarters cooked.
While the rice is simmering, heat the oven to 130C fan/gas 2, then pour the rice into a baking dish and bake for 30 min. Leave to stand in a warm place for 1-2 hours. Sprinkle with icing sugar.
To create a caramelised top, heat the grill to high and sprinkle the extra 50g caster sugar evenly over the surface of the pudding using a sieve. Place under the grill for 2-3 min to caramelise, then leave to cool for 10 min before serving.