Slowly heat the milk and vanilla pod (bean), halved lengthwise. When almost boiling, remove the pan from the heat and set aside to infuse for about 20 minutes. Beat the egg yolks, sugar and salt together, until the mixture is very pale and the whisk leaves a trail. Slowly add the strained warm milk, whisking all the time. Return the mixture to the pan and cook over a very low heat, stirring constantly, until the custard is thick enough to coat the back of wooden spoon. Set the custard aside and stir it occasionally while it cools. Add the vanilla essence (extract), if using Pour into freezer trays and still freeze, whisking the mixture once or twice during freezing