This is an old-fashioned recipe, at least 50 years old, from a time when people wanted a nice treat but did not make them as huge as we do now. The lemon custard layer on this square is thinner than the lemon bars you may be used to getting at your local bakery. You can double the ingredients for the filling to make the custard layer twice as thick if you wish. You will need to increase the cooking time by 5-10 minutes. So check at the 25-minute mark and check again every 3-5 minutes until it feels set. However, I've always found this recipe to be perfect as is.