To make the caramel, put the water and caster sugar into a pan and put it on a medium heat. Cook until it melts and forms a dark golden-brown caramel. Don’t go anywhere while this is happening. Pour ½cm of caramel into each of six ramekins and leave to cool down.
Put the milk into a pan on a low heat with the double and single creams and the split vanilla pod and leave to infuse. Don’t let it boil.
Whisk the eggs and egg yolks with the sugar until pale and creamy and the sugar has dissolved. Add the infused milk bit by bit, then pour into the ramekins.
Heat the oven to 160–170°C. Stand the ramekins in a deep roasting tray, then pour hot water into the tray to come halfway up the sides of the ramekins and make a bain-marie. Cover with a flat tray or foil, to stop the tops from colouring. Cook for 18–20 minutes: check it’s cooked by taking out one of the ramekins and giving it a little push. The flan should wobble but hold together. If it’s still loose, give it another 4–5 minutes but keep an eye on it: if the flan comes away from the ramekin edge, it’s overcooked.
Leave to cool, then refrigerate – it will need 3–4 hours to become cold and set properly. To turn out, fill a bowl with some warm/hot water and dip a ramekin in it for a couple of seconds. Press around the flan with your fingers, or cut round it with a small sharp knife to loosen. Invert the ramekin tightly on to a plate and give it a swift sharp shake to the side: you want to hear a ‘bloop’ kind of sound, indicating that it’s come out – like turning out a jelly.