Tip the sugar, butter, milk and evaporated milk into a heavy based pan and heat gently, stirring frequently, until the sugar has dissolved.
Then bring to the boil without stirring, and continue boiling until a temperature of 116°C (240°F) is reached on a sugar thermometer. If you don't have one of these, you can test if the fudge is at this temperature by spooning a small amount of the syrup into some iced water, it should form a soft ball.
As the temperature rises, stir the fudge occasionally (be careful as the mixture is very hot) so that the sugar doesn't stick and burn. Remove the pan from the heat, add the vanilla extract and beat the mixture with a wooden spoon until thickened.
Pour into an 18cm (7in) shallow square tin brushed with a little vegetable oil, and leave for 10-15 mins or until almost set. It will set quicker in the fridge.
Mark the fudge into 36 squares with a sharp knife and leave to cool completely.