Combine the custard powder with the sugar. Add 2-4 tbsp coconut milk from a 400ml can and mix to a smooth paste. Heat the rest of the coconut milk from the can in a pan with another 400ml can coconut milk and the water until almost boiling. Whisk some of the coconut milk mixture into the custard powder mixture until it looks like a smooth custard sauce, then tip this into the pan with the rest of the hot coconut milk. Return to the boil over a low heat, stirring continuously, until you have a thick, smooth custard.