Preheat the oven to 180 degrees celsius (fan-forced). Place the pastry sheets on flat baking trays lined with baking paper. Use a fork to prick holes all over the pastry. Add an extra sheet of baking paper on top of each pastry sheet and place an extra baking tray on top as a weight.
Bake for 20-25 minutes or until golden. Allow to cool completely on a wire rack.
Place a 20cm square baking tin on top of the cooked pastry sheets and use a serrated knife to carefully trim the sheets to the size of the tin.
Grease and line the 20cm square baking tin with baking paper (allowing it to overhang). Place one of the pastry sheets in the base of the tin and set aside.
Place the custard powder into a saucepan with ¼ cup of the milk. Stir until all of the custard powder has dissolved. Add the remaining 2 ¼ cups of milk and the caster sugar. If using a Thermomix: Place the custard powder, milk and caster sugar into the Thermomix bowl. Cook for 9 minutes, 90 degrees, Speed 4. If the custard is too runny, cook for a further minute.
Whisking continuously, heat the custard over a medium heat for 5 minutes or until it has started to thicken. Once the custard starts to boil, continue whisking for a further 2 minutes or until very thick. Place a sheet of glad wrap over the custard to stop it forming a 'skin' and allow to cool for 20 minutes.
Spread the cooled custard over the pastry and top with the extra sheet of cooked puff pastry.
To make the icing, sift the icing sugar into a bowl and add the food colouring. Add the water 1 tablespoon at a time and stir until you have a smooth paste (that can be spread). If using a Thermomix: place the icing sugar into a clean and dry Thermomix bowl. Sift by pressing Turbo for 2-3 seconds. Add the pink food colouring and 3 tbs water. Mix for 10 seconds, Speed 6. Scrape down the sides of the bowl and mix for a further 5 seconds. If the mixture is too thick and can't be spread, add 1 more tablespoon and mix again (it should be a thick paste).
Spread the icing over the slice and place into the fridge for 3-4 hours (overnight is best).
Use a small, serrated knife to cut through the top pastry sheet before using a large, flat knife to press down firmly to cut into slices.
Notes
RECIPE NOTES & TIPS
Use a fork to prick holes over each pastry sheet before baking, to make it extra crispy.
Bake each pastry sheet between two sheets of baking paper and two baking trays to cook flat and avoid it puffing up.
It's important to continue to cook the custard (while continuously whisking) until it is very thick - this will ensure it sets firm.
Glaze - only add water a little at a time, just enough to make the glaze spreadable. Too much water will make the glaze too runny, and soften the pastry on top which you want to stay crispy and flaky.
Pink food colouring – optional. You only need a teeny-tiny drop of food colouring – don’t overdo it!
Passionfruit icing - to make passionfruit glaze, omit the pink food colouring and water and replace with passionfruit pulp (use just enough to get a spreadable thick paste).
Cutting the slice - for best results, leave the slice in the fridge overnight and cut whilst still in the tin. Use a small, sharp serrated knife to cut through the top layer of the puff pastry. Then use a large flat knife to press through the custard and base layers.
Storing - vanilla custard slice can be stored in the fridge for 3-4 days, however the pastry layers will soften the longer the slice is in the fridge.
Freezing - this slice is not suitable for freezing.