Preheat the oven to 150C/300F/Gas 2. Pour the cream and the milk into a heavy-bottomed pan over a medium heat and bring to a simmer.
Split the vanilla pods down the middle with a sharp knife. Scrape out the seeds with the knife and add the seeds and the pods (or the vanilla extract) to the cream and milk mixture. Stir well to combine.
Separate the egg yolks and egg whites. Only the yolks are used in this recipe, so you can use the egg whites for another dish or freeze them for future use.
Pour the caster sugar into the bowl with the yolks. Whisk together the egg yolks and the caster sugar until well combined.
When the cream and milk mixture is coming up to a simmer, pour it (including the vanilla pods, if using) into the bowl with the egg yolk mixture. Stir thoroughly for a minute or two to dissolve the sugar. At this stage, you can chill the mixture in the fridge and make the brulees the following day, or carry on with the recipe.
Ladle the brûlée mixture into the serving dish (or dishes). Discard the vanilla pods.
Place the crème brûlée dishes into a deep baking tray and pour hot water into the tray, until the water reaches halfway up the sides of the dishes (this is called a bain-marie).
Place the bain-marie into the preheated oven and cook the creme brulees for about 30-35 minutes, until set firm but still with a slight wobble. Allow to cool. You can place the brûlées in the fridge at this stage if you're not serving them immediately.
For the topping, sprinkle half of the remaining caster sugar on top of the brûlées. Heat the surface with a mini-blowtorch until it forms a thin layer of caramel.
Sprinkle over the rest of the sugar in a final layer and caramelise with the mini-blowtorch once again, to form a thick crunchy layer. The crème brûlées are ready to serve immediately or in 2-3 hours.