In a medium saucepan, add cream, milk and sugar. Whisk until the sugar has dissolve and bring to a light simmer. Once simmering, remove from the heat.
In a medium bowl, add the egg yolks, vanilla essence together. In a separate bowl, mix the cornstarch and water together until the cornstarch has dissolved. Add the cornstarch mixture to the egg and whisk well.
While whisking the egg mixture, gradually pour in the warm milk mixture until full incorporated. Pour the mixture back into the medium saucepan and bring to a medium heat. Whisk until the custard starts thickening. You want it to be very thick, about 1-2 minutes. Add the butter and salt and vigorously mix until well incorporated.
Pour the custard on top of the the first layer of baked pastry and spread evenly. Place the second pastry on top. And gentle set it in place.