You will need to start the recipe at least on day before so the custards have time to chill.
Place the broken-up chocolate, along with 150 ml of the cream in a large heatproof bowl sitting over a saucepan of barely simmering water, making sure the bowl doesn't touch the water.
Then, keeping the heat at its lowest, allow the chocolate to melt slowly – it should take 5-6 minutes. Remove it from the heat and give it a good stir until it's smooth and glossy, then remove the bowl from the pan and let the mixture cool for 2-3 minutes.
Then place the remaining cream in a pan over a gentle heat and heat it to just below simmering point, stirring occasionally with a wooden spoon.
While the cream is heating, use a balloon whisk to whisk together the egg yolks, sugar and cornflour mixture and the vanilla in a medium bowl with a cloth underneath to steady it. Then, whisking the egg mixture all the time with one hand, gradually pour the hot cream into the bowl.
When it's all in, immediately return the whole lot back to the saucepan using a rubber spatula. Now back it goes on to the same gentle heat as you continue whisking until the custard is thick and smooth, which will happen as soon as it reaches simmering point.
If you do overheat it and it looks grainy, don't worry, just transfer it to a jug or bowl and continue to whisk until it becomes smooth again.
Now whisk the chocolate mixture into the custard then divide the custard among the ramekins and leave to cool. Then cover each dish with clingfilm and refrigerate overnight.
To caramelize the tops, first pre-heat the grill to its highest setting then uncover the ramekins and sprinkle each of the tops evenly with one tablespoon of granulated sugar.
Place them on a baking tray and put them under the hot grill for 4-5 minutes until the sugar has melted and started to go very dark brown in some places.
Leave the caramel to harden before serving or you can freeze them as they are in the Iced Chocolate and Vanilla Creme Brulee recipe below