Place the milk (reserving about a ¼ cup of milk to mix the custard powder in - see below) and butter in a sauce pan, and place on a stove top on a medium heat.
While the milk is coming to the boil, mix the custard powder into the reserved ¼ cup of milk, stirring with a fork until it is lump free, and set aside.
In a mixing bowl, beat the eggs until they are fluffy. Then add the custard powder and milk mix, and the condensed milk, and give a quick beat to combine.
When the milk on the stovetop begins to boil, add the egg, custard & condensed milk mixture to it, while stirring, and continue to stir until it gets thick (roughly 3 minutes).
Pour the custard into the dish lined with the puff pastry, and top with the other half, top side facing up.
Place in the fridge to get cold. After about an hour, you can mix the icing for the top.