Bring the milk to a simmer in a heavy-bottomed saucepan. Meanwhile, in a mixing bowl, stir together the sugar, cornstarch and salt. Gradually add the hot milk to the sugar mixture, whisking constantly. Pour the milk mixture back into the saucepan and cook over medium heat, stirring constantly, until the mixture boils and thickens slightly, 7 to 10 minutes. Whisk in the egg yolks, then cook, stirring, until slightly thicker, about 2 minutes more. Remove the saucepan from the heat and stir in the butter.
Let cool to room temperature, stirring occasionally. Add the vanilla, cover and refrigerate until thoroughly chilled. Pastry cream will keep in the refrigerator, tightly covered, for up to 3 days.
Makes about 1 1/2 cups.
Williams-Sonoma Kitchen Library Series,
Pies & Tarts,
by John Phillip Carroll
(Time-Life Books, 1992).