Warm the milk, infusing the lemon rind if used. Mix eggs and sugar to a liquid. Pour the warmed milk over the eggs and strain the custard into a pan previously rinsed with cold water. Cook the custard gently until the eggs have coagulated and thickened the milk. To ensure that this occurs evenly and forms a smooth creamy texture, stirring should be brisk and thorough. If the custard is cooked in a saucepan, a wooden spoon will be found most suitable for this, as the thick edge of the spoon works smoothly over the base of the pan, keeping it clear. If the custard is cooked in a double saucepan over hot water, a whisk is better, as thickening takes place from the sides as well as from the base. Do not let the custard boil. When the custard coats the spoon, pour into a cool bowl, and add the vanilla, if used. Add the cream (if used) and stir in lightly. Stir frequently during cooling so that a skin does not form on the surface.