Preheat oven to 140C. Put 250ml double cream and 75ml full fat milk into a pan and bring to the boil. In a jug, whisk 4 egg yolks and 3 tbsp caster sugar until pale and thick. Add 1 tsp vanilla extract and pour into the boiling cream mixture. Stir and then pour the mixture into a big ramekin. Then place in a baking tray. Fill the tray with two inches of boiling water and cook for 20 minutes.
Remove and cool. Sprinkle with an even layer of sugar. Put under a very hot grill until you have a golden topping. Leave to cool and serve.