Pour milk into top of a double boiler; bring water to a boil. Heat milk until tiny bubbles begin to appear around edges of pan. Remove from heat, and set aside.
Beat egg yolks with a wire whisk until frothy. Add sugar, cornstarch, and salt, beating until thickened. Gradually stir about 1 cup hot milk into yolk mixture; add to remaining milk, stirring constantly.
Cook custard mixture in double boiler over low heat 25 minutes or until mixture is thickened and a candy thermometer registers 180°, stirring occasionally. (Do not boil.) Stir in vanilla. Serve warm or cold; sprinkle with nutmeg.