Rub the kunafa pastry with the butter, then spread a layer in a baking dish.

In a saucepan, combine the milk, sugar, cornstarch, and custard powder. Stir constantly over medium heat until the mixture thickens, then set aside to cool slightly.

Once the custard mixture has cooled a bit, pour it over the first layer of kunafa. Cover with another layer of kunafa pastry, then bake at 350°F (180°C) for 35 minutes, until golden on top.