Custard pies are tricky. You have to hit the sweet spot in temperature when cooking to keep the custard from breaking down.
I know a lot of recipes instruct cooking the custard until it comes to a boil or coats the back of a spoon.
But a custard should be heated to boiling, which is 212°.
Custards should be heated between 160° to 180° max. Going over 180° will make the custard watery. The exact temperature depends on the recipe (amount of egg, amount and type of thickener) of your recipe. Try heating the custard to 165°.
The reason custards are not boiled is the heat tiggers denaturation of egg proteins, which is the change that causes eggs to coagulate. Denaturation begins around 150°. If taken above 180°, the pressure from coagulation forces the water out of the egg.
That’s what causes a custard to curdle when it’s heated over boiling.
Whether you cook the custard on the stove top or you bake it in the pie shell like a pumpkin pie, you have to monitor the temperature.
A lot of custards use cornstarch as a thickener. The starch allows the custard to be heated to a higher temperature. But there is a thermal limit for cornstarch as well. Cornstarch thickens at around 180°. Cornstarch can go to the boiling point of 212°. But prolonged heat will cause it to break down and loses all of its thickening ability. The cornstarch becomes too weak to hold onto the water that it absorbed in the custard. Same thing happens when cornstarch is used in pie filling.
So both the egg and cornstarch contribute to a runny custard pie.
Whipping cream is about 36% fat. The rest is nearly all water. That’s why it is extremely unstable. After it’s whipped it immediately starts to break down. It’s best to whip cream just before serving. If you’re going to use it as a topping or a cake filling, then it’s best to stabilize it. But even stabilized with cream will not hold up for long. Cooks Illustrated has a reliable recipe for stabilized whip cream.
https://www.cooksillustrated.com/how_tos/8351-stabilizing-whipped-cream
