Tip the sugar into a small saucepan, add the measurement water and cook over a low heat until the sugar has dissolved, then increase the heat and cook until the sugar turns dark brown. Pour a little of the liquid into each of 6 individual metal dariole moulds and leave to cool.
Whisk together the condensed milk and eggs in a bowl, then add the milk and vanilla extract and stir together until well combined. Pour the mixture into the dariole moulds and place in a deep baking dish. Pour in boiling water until it comes halfway up the sides of the moulds.
Place in a preheated oven, 160°C (325°F), Gas Mark 3, for 15–20 minutes until just set but still with a slight wobble at the centres. Leave to cool, then chill for at least 4 hours or preferably overnight.
When ready to serve, run a knife around each mould, invert on to a serving plate and then give a gentle tap to remove the flan from the tin. Drizzle over any remaining caramel.