In a small bowl, mix together the custard powder and 1/2 cup of the milk; set aside. Sprinkle the sugar into a saucepan over medium heat. When the sugar begins to caramelize, add the remaining milk and coffee to the pan, and bring to a boil. At this stage, the caramel will become very crusty. Reduce heat to low, and cook until the sugar dissolves, about 5 minutes. Stir in the custard powder mixture, and cook, stirring until the custard thickens. Remove from heat, and whisk in the egg yolk.
In a clean glass or metal bowl, whip the egg white until soft peaks form. Fold the egg white carefully into the custard mixture. Transfer to a serving dish and refrigerate until firm.