This recipe uses no custard powder. Instead the custard is made from scratch from milk, butter, cornflour, icing sugar and eggs. It's sandwiched between layers of puff pastry.
the custard was delicious but the making of the square is not as easy as it sounds. The puff pastry over puffed, needed to allow more time for the custard to cool, and the volume of custard made does not fill the square as full as the picture - ours ended up more the reverse of the image with barely visible custard to giant pastry will make another attempt as it was very yummy. - 11 Nov 2012
I would tend to agree with foufee in that the filling would be insufficient. And why bother to avoid custard pwwder when it's made from cornflour anyway? I would also like to add that directions to prick the pastry aren't adequate. You need to use a sharp stabbing force so the times of the fork go right through the pastry. And the holes need to be very close together. Once I started doing this instead of just pricking the pastry I had no problem with it puffing. For two ready rolled pieces of pastry I find I need four cups of milk. Last time I made custard squares I also used three eggs and I've been wondering about tryin four. - 15 Jul 2015