Before you start, preheat oven to 200°C (or 180°C fan bake). Line a square cake tin with baking paper (about 22cm).
Line an oven tray with baking paper. Place a piece of pastry on the tray and top with another layer of baking paper. Cover with a second baking tray, so the pastry is sandwiched between trays.
Bake pastry for 15-18 minutes, until golden. Repeat with second piece of pastry. Set aside to cool completely.
Place egg yolks, sugar and cornflour in a large bowl and whisk thoroughly to combine.
Place milk, cream, vanilla and salt in a large pot and bring to a simmer on medium heat. Once simmering, pour half of the milk mixture into the egg mixture and quickly whisk to combine. Pour in the remaining mixture and whisk thoroughly.
Transfer mixture back to pot and place on medium heat. Stir continuously with a spatula, making sure nothing sticks to the bottom, for about 6 minutes, until very thick and custardy. Stir through butter, until completely combined.
Once pastry has cooled, trim it to fit the size of your square tin. Place on sheet in the bottom, cover it with the custard, then place the remaining pastry sheet on top. Place in fridge to set for at least 3 hours, or overnight.
Sift icing sugar into a medium bowl. Slowly whisk in water measure, until thick and spreadable. Spread onto pastry top and return to fridge for 10 minutes to set.
To serve, remove custard square from cake tin. Using a bread knife and a sawing motion, cut into smaller squares for serving.
TIP Pastry can be baked and trimmed the day before and stored in an airtight container until ready to assemble.