Working with Cream Pies
To prevent the filling in a cream pie from becoming thin and watery as it cools in the baked pie shell, be sure to cook the filling sufficiently after the egg yolks are added to the rest of the mixture. Failure to do this often results in the pie filling breaking down upon cooling.
For best results, combine the sugar, cornstarch or flour in a heavy saucepan; gradually add milk. Cook and stir over medium-high heat until thickened and bubbly. Reduce heat; cook and stir for 2 minutes. Remove from the heat and add about 1 cup hot liquid to the beaten egg yolks. Return all to the pan; cook and stir 2 more minutes. Pour into baked pie shell.