To make the caramel, melt the caster sugar in a medium saucepan until it bubbles and darkens, a little darker than golden syrup. Take off the heat and stir in the water, be careful…it will spit! Stir until smooth and pour to coat a 700ml baking dish, swirl round the sides.
To make the custard, heat milk and cream in a saucepan until steaming hot. Meanwhile whisk the brown sugar, eggs and vanilla in large bowl. Whisk the steaming hot milk into to the mixture.
Pour custard mixture into the baking dish. Place the dish in a larger dish in oven and pour hot water into the outer dish to make a 'bath'.
Bake for 1 hour in the preheated oven, or until the custard has an even jiggle.
Cool to room temperature, then chill in refrigerator, preferably overnight. Run a knife around the edge, give it a little shake to loosen. In one movement (with plate over top of dish), turn it out to serve.