I'm making a lemon bars/squares recipe that I've made a few times before, and I keep having trouble deciding when to take it out of the oven. I don't want to overbake it, but I don't want to underbake it either as I won't be able to cut it properly into squares. The recipe says "until set, about 25-30 minutes". My lemon bars have been in the oven over half an hour; the top has gone light brown, but it still jiggles around like liquid when I move it. How do I know when it's "set"?
Custard without any flour or starch should be baked to about 83 Celsius internal temperature. Below 80 is underbaked, above 90 it curdles, so you have a very small window of good texture. It may be even narrower, it depends a bit on the other stuff you have in there, sugar gives you wiggle room but acids (including lemon juice!) will make it harder to get right.
If you have starch or flour, you need to reach almost boiling temperatures. 95 Celsius should be OK.
If you want high confidence, you really need to measure the internal temperature. External signs are not reliable, and time even less so.
Custard is always sort of jiggly, even when it's "set"; furthermore, it's one of those dishes that sets properly only after you remove it from the heat of the oven. The link you gave in the comments mentions a thumbprint test for doneness:
Bake 25 to 30 minutes or until no indentation remains when touched lightly in center.
Next time try that, and see if it sets to the desired consistency when cooling. Browning is not usually desired for this type of dish, so I'd say 45mins is too long :)
Note: The part that still baffles me is that you mentioned it still being liquid when cut into. That's definitely not supposed to happen, and I'm not sure what went wrong there, so perhaps try a different recipe next time?
It can also be something related to the size of the dish. I always use an 8x8 Pyrex. I was at a friends house the other day and didn’t have one so I doubled the recipe, without thinking about the fact that 8x8 is (64square inches) and the dish i was using was maybe 9x12 (108square inches), so when I poured my mix into the pan I realized that they were much thicker than usual. Because of this they were nowhere near done in the usual baking time. I left them longer and the top turned a golden brown (which was ok, I scraped it off with a knife immediately and it basically peeled off like a giant band-aid) however the center was still liquid while everything else was done so I turned down the temperature and placed them back in the oven for another 10 minutes and they actually came out fine. Not perfect mind you, but they were definitely ok. Better than what you would find at a hotel or casino buffet.