For the custard filling: Prepare a bowl that will accommodate some ice water AND the eventual finished cooked pastry cream bowl.
In a saucepan, whisk together the sugar, cornstarch and salt until combined. Add about half the milk and whisk until smooth. Whisk in the eggs, then the remaining milk. Heat the pan while stirring over medium high heat to a full boil. Drop the heat to medium and whisk for 1 minute (it will thicken rather quickly!) Off the heat, add the vanilla and whisk in the butter a couple lumps at a time until smooth, then chill saucepan in the ice bath you had ready.
Cover the pastry cream so plastic wrap touches the surface of the cream but peel it back ¼” on one of the sides to allow steam to escape and prevent a ‘skin’ from forming. Store cooled pastry cream in the refrigerator for up to 2 days.
For the pie:
Preheat oven to 350F. Make sure the pie crusts have been out of the fridge for 20-30 minutes. Give a couple of spritzes of cooking spray and a sprinkle of flour to a 9” pie dish. Lay in the first crust and let the sides hang over the dish. Whisk down the cooled pastry cream from above and spread evenly over the bottom crust and smooth flat with a small offset spatula or spoon. Top with the second pie crust, pinch together and eliminate any air pockets. Trim any excess and decorate the edge to your liking. Brush a beaten egg on top and sprinkle with a little sugar. Bake off at 350 for 35-40 minutes until you get the desired crust color. Let cool before service.