Why would pumpkin pie form a clear custard layer? My PA Dutch grandma's pumpkin pie always had a thin layer of  custard on the bottom.  We have her recipe but none of us has been able to get the filling to separate... Anyone have any idea what might cause this?  We've experimented with cooking temps, ingredient temps, amount of mixing, beating the eggs more or less etc....  Any ideas????
I believe that your aunt is right and Grandma added cornstarch. Quite a few times in the past when I've made sauces using cornstarch in a saucepan, bubbles of clear gel have formed when I've neglected to stir constantly. It seems quite possible to me that a thin layer of this gel would form when making a pie, rather than the bubbles that I have seen.

I understand that you want to come as close to Grandma's recipe as possible and keep this layer. Bearing in mind that I've not tried this, I would mix 1 to 2 tablespoons of cornstarch with just enough milk to make a pourable, non-lumpy paste and whisk it gently into the custard  . If the recipe involves any heating of the custard before baking, be sure to incorporate the cornstarch mixture only after heating.