Combine all ingredients through margarine in a heavy saucepan with a candy thermometer. Mix and bring to a boil over medium heat. Keep stirring intermittently.
2. Put the milk, sugar and butter in a heavy-based saucepan. Heat slowly, stirring all the time, until the sugar has dissolved and the butter melted.
3. Bring to the boil and boil for 15-20 minutes, stirring all the time.
4. When the mixture reaches the soft-ball stage (115°C on a sugar thermometer), remove from the heat and stir in the vanilla extract. Leave to cool for 5 minutes.
5. Beat the mixture with a spoon for a few minutes until it starts to thicken and the gloss disappears.
6. Pour into the prepared tin and leave to set at room temperature (do not put it in the fridge).
7. Once set, cut the fudge into small squares and store in a sealed container.