To prepare before cooking begins: Preheat the oven to 210 degrees, and lne either a square pan or a slice tray with aluminium foil. Make sure the foil comes out over the side so that you can lift the slice out when finished.
To make the pastry layers: Place each pastry sheet onto a baking tray layered with non-stick baking paper. Bake these at 210 for 10 minutes until golden brown. Set aside on a cooling rack to cool.
To make the custard filling:
Place the caster sugar, custard powder and cornflour into a bowl and mix to combine. Set aside, ready to add the bowl's contents into the milk once heated.
Weight out the butter into a separate bowl. Also place the egg yolks into a separate bowl and use a fork to lightly beat. Also set both these bowls aside, so that everything is prepared and all you have to focus on is mixing the custard.
Place the milk in a pan over medium heat. Use a whisk to mix through the vanilla essesnce and warm gently.
Remove the milk pan from the heat and place firmly on a bench. Whisking constantly with one hand, gradually pour the cornflour, custard powder and caster sugar into the milk. Whisk until well combined then return to medium heat.
Stir constantly until the mixture thickens. When the mix thickens to about half way, you may want to switch from the whisk to wooden spoon.
Add the butter and stir well to combine.
Remove from heat and whisk through the egg yolks until smooth.
To assemble: Place 1 prepared pastry sheet in the bottom of the prepared pan. Pour the custard filling on top (making sure it is spread evenly), then place the second pastry sheet on top. Refrigerate to set.
Ice by combining the vanilla essence, icing sugar, water and butter, then spreading the icing over the top pastry shell.