Heat the butter in a wide, shallow saucepan until starting to sizzle, then add the rice. Toast the rice gently for about 4 mins in the butter until just starting to turn light brown, then scatter over 100g of the sugar and cook for a few mins more until the sugar is starting to dissolve.
Pour in the cream, stir and boil until all the sugar has dissolved into a thick butterscotch sauce, then gradually stir in 500ml of the milk and the vanilla extract. Simmer everything gently for about 45 mins, stirring often. If towards the end of the cooking time the liquid is too thick and the rice isn’t cooked yet, add another 100ml milk and carry on cooking. The finished consistency should be like a really creamy risotto.
When the pudding is cooked, tip it into a shallow, heatproof serving dish. Scatter the rest of the sugar over the top, and grill or blowtorch until sizzling and just starting to burn. Leave the sugar to form a crisp layer and serve hot or cold straight from the dish.