Preheat the oven to 160°C/325°F/Gas mark 3. Brush eight 125ml/4fl oz ramekins or moulds with melted butter.
Put the granulated sugar and 50ml/2fl oz water in a pan. Stir over a low heat until the sugar has dissolved. Bring to the boil, reduce the heat and simmer until the mixture turns golden and starts to caramelize. Remove from the heat immediately, and pour enough hot caramel into each ramekin to cover the bottom.
To make the custard, heat the milk in a pan over a low heat until almost boiling. Remove from the heat. Whisk together the caster sugar, eggs and vanilla essence for 2 minutes, then stir in the warm milk. Strain the mixture into a jug, and carefully pour into the ramekins.
Put the ramekins in a baking dish, and pour in enough boiling water to come halfway up the sides of the ramekins. Bake for 30 minutes or until the custard is set. Allow to cool, and refrigerate for at least 2 hours before serving.