I always make my own custard when I bake Portuguese Tarts but want to take a shortcut. Can I use store-bought custard to make Portuguese Tarts? What kind of custard do I need to buy? Should it be at room temperature before I bake?
Short Answer: Probably, but it won't taste as good.
Longer Answer: Given that most packaged custards (if you mean the powdered sort such as Bird's or Jello) do not contain eggs, you might find that the custard does not behave quite the same and certainly will not taste as rich. They tend to use yellow food colouring to give the bright yellow colour that you would get from the egg yolks. It's a bit like asking if you can substitute Cool Whip for Whipped Cream. Yes, but....
If by store bought custard you mean something like Bird's custard powder, then no you cannot. Though I don't believe there is a hard distinction between them, generally what the English consider custard is more of a cream. I make the distinction, again this maybe somewhat arbitrary, in that a custard is intended to be cooked and set, which is what you're going for with the Portuguese tarts. A cream, on the other hand, is constantly stirred while heating to prevent setting, such as creme anglaise (which is what Bird's and other custard powders are intended to mimic).
Looking at this I would suggest that as opposed to a 'custard' that what would work, and taste better would be a store bought creme patisserie
In the uk where I am I would suggest either waitrose or marks and Spencer's
If you live abroad and can't get this then several uk food exporters sell around the world, including this one
(Other retailers also carry this)