Rinse the pan in hot water and dry before pouring the custard mixture back in. Cook over a low to medium heat whilst constantly stirring with a spatula. After about 10 minutes, the custard should thicken (this can be tested by making sure it coats the back of a spoon). Remove from the heat and set aside to cool, stirring every now and again to avoid a skin forming. Once cooled, cover with clingfilm and chill in the fridge.