Put the puff pastry in a rectangular tray. Bake the 2 layers from the packet separately.
For the cream: First make the starch cream - boil the milk, add the sugar. Pour the starch in a thin trickle after dissolving it in a separate bowl of water.
Separate the egg whites from the yolks. Beat the whites to fluffy snow. Beat the yolks so you can add them in a thin trickle to the starch cream.
Once the starch cream has cooled, add the yolks and butter to it and beat with a whisk by hand. Finally, add the egg whites carefully, so they don't lose their volume.
The cream needs to be very well cooled, otherwise it'll change the consistency of the puff pastry.
Cut the cooled puff pastry layers into 2, for a total of 4. Arrange puff pastry layer, cream, layer, cream, layer, cream, layer.