Bring the milk to a simmer in a heavy-bottomed saucepan over medium heat and add the vanilla.
In a bowl, whisk together the sugar, egg, egg yolks, and cornstarch until smooth. When the milk comes to a simmer, stir half of it into the egg mixture. Pour the egg mixture back into the saucepan containing the rest of the milk, place over medium heat, and stir the mixture with a whisk or wooden spoon. Reach into the corners of the saucepan so the pastry cream doesn't hide out there and scald.
When the pastry cream comes to a boil and thickens, transfer it to a bowl and cover with plastic wrap touching its surface so a crust doesn't form. Refrigerate for up to 4 days. To loosen pastry cream that has gotten stiff in the refrigerator, whisk it by hand or in a mixer.

VARIATIONS
Methods for lightening Pastry Cream:
To lighten pastry cream with whipped cream, combine one part whipped cream to three parts pastry cream. Whisk one-fourth of the whipped cream into the pastry cream and then fold the pastry cream with the remaining whipped cream. If you're using this mixture in something that's not going to be served immediately, use stabilized whipped cream.
To lighten pastry cream with Italian meringue (creme chiboust), combine equal parts Italian meringue and pastry cream. Whisk one-third of the Italian meringue into the pastry cream and then fold the pastry cream with the remaining Italian meringue. You can also add 1 tablespoon brown butter to each cup of this mixture to give it a nutty flavor, or you can flavor with spirits such as kirsch.