Method

1. Preheat the oven to 200°C.  Unroll the 3 rolls of puff pastry.  Place each on a baking tray that has been lined with foil.  Bake for 15–20 minutes until golden brown.  Lift off the baking tray using the foil. Place the first piece into a rectangular dish about 22cm x 30cm.
2. For the filling: Mix the 250ml milk, cornflour, custard powder, castor sugar and salt together.
3. Heat the cream and 625ml milk in a saucepan until hot.  Slowly stir in the custard mixture.  Stir over heat until it thickens and is smooth.
4. Remove the custard from the heat.  Separate the eggs and stir the egg yolks, butter and vanilla essence into the thick custard mixture.
5. Beat the egg whites until stiff and fold into the custard.
6. Spoon half of the custard over the pastry in the dish.  Place the second piece on top.  Top with remaining custard and the last piece of pastry.  Refrigerate overnight. 
7. Mix icing sugar and boiling water together until smooth.  Spoon over the pastry and serve sliced.