Simmer milk for 5 minutes; stir in vanilla extract. Combine sugar, egg yolks and salt, and beat until fluffy and lemon-coloured. Pour a little of the hot milk into the egg and sugar mixture; blend well, and then stir into the hot milk. Heat slowly in the top of a double saucepan, stirring constantly, until the mixture coats a spoon. Serve warm over cake, sweet soufflé or ice cream.