Preparation

Bring milk and sugar to a simmer in a saucepan without stirring. Whisk yolks in a bowl until blended, then gradually whisk in hot milk mixture. Return to saucepan and cook over moderate heat, stirring constantly, until sauce thickens enough to coat back of a wooden spoon and registers 170°F. on an instant-read thermometer.

Immediately pour sauce through a fine sieve into a metal bowl and place bowl in a large bowl of ice and cold water. Stir in vanilla and whisk custard occasionally until cool, then cover surface with a piece of wax paper to prevent a skin from forming.

Chill until cold.