Place 1 sheet of puff pastry on flat baking tray, top with baking paper and place a second baking tray on top to prevent pastry from puffing.
Cook pastry for 15 minutes or until nicely browned, then repeat with second sheet of pastry.
When pastry has cooled, place one sheet in a square 23cm cake tin, trimming to fit.
Place milk, cream, caster sugar, custard powder, cornflour and vanilla in a heavy based saucepan. Bring to the boil, stirring until thickened, about 10 minutes.
Add egg yolks and butter, stirring well.
Pour custard on pastry sheet in tin, and top with second pastry sheet. Cool in the fridge then ice or dust with icing sugar. Cut into 9 squares. Best eaten within one day.
Combine icing ingredients, adding only enough milk to make spreadable.
I used a plastic cake container with a lid and lined it with cling wrap, leaving an overhang so I could lift the slice out of the container nice and easy. Once you've taken what you want, put the lid on it and put it back in the fridge.