Originally Posted by ChefJune
I'm guessing your oven might be a bit off. Get an oven thermometer and check that out. Overcooking is the first thing I thought of.
That often also happens with cheesecake. Things continue to cook from the inside out after you remove them from the oven. If you think the middle looks a bit undercooked, go ahead and pull them anyway. The residual heat will finish the job, and you may eliminate the cracks.
That's what I thought. I always thought that cracking in custards (or those recipes that are proportionately higher in egg) was caused by overcooking/ too hot/ too fast cool-down. It has something to do with the proteins in the eggs. At least that's what I remember reading. I, too, take my cheesecake/crème brûlée/custard bars out when they look "not quite done". Actually, I turn off the oven and slide a wooden spoon in the door to gently "vent" the heat out with the dish still inside.
Perhaps it's a combination of things. You could try to change the cooking temp/time first, then tweak the recipe to add moisture if that doesn't work. Or visa versa. Good luck. I know the cracks don't hurt anything, but they say you eat with your eyes first.