Rinse out a saucepan with cold water, to lessen the risk of the milk burning. Then pour in the milk, and bring it gently to boiling point. When it boils shake in the semolina, stirring it all the time. Let it boil gently until the semolina is quite clear and the mixture thick. Keep it continually stirred. It will probably require about eight minutes to cook it. Add the sugar and vanilla to taste. . Then pour the mixture into a mould that has been rinsed out with cold water, as when poured into a wet mould it is more easily turned out. Leave until cold, then loosen the edge round the top and turn the mould carefully out on to a dish. Serve with stewed rhubarb, or with jam and custard round if preferred. If the mixture is too thick to pour smoothly into the mould, add a little more milk.