1. Cut the pudding or cake into fingers, and arrange in a greased 750 ml pie dish.
2. Beat the egg until liquid.
3. Mix the cornflour to a paste with a little of the milk.
4. Heat the remaining milk to scalding point, then pour it slowly on to the cornflour, stirring to prevent lumps forming.
5. Return it to the heat and cook gently for 2 minutes; then stir in the sugar, egg, and rum or rum essence.
6. Pour the mixture over the pudding or cake and bake in a moderate oven at 180°C, Gas 4, for about 30 minutes or until firm.
7. Serve with Fairy Butter