Butter an 18 cm square, shallow tin. Pour the water into a heavy-based saucepan, add the butter, caster sugar and golden syrup and stir over low heat until all the sugar has dissolved. Brush down the sides of the pan with hot water from time to time. Add the condensed milk, bring to the boil and cook, stirring continuously, for about 10 minutes, or until the mixture reaches a temperature of 118°C on a sugar thermometer.
Alternatively, take the pan off the heat and test by dropping a teaspoon of the mixture into a bowl of cold water – if it does not form a soft ball when taken out, continue to boil for a little longer and retest as before.
When the fudge is ready, remove the pan from the heat and set aside until the bubbles subside. Stir in the vanilla extract and beat with a wooden spoon until the texture is thick and creamy and a heavy trail is formed when the mixture is allowed to fall from the spoon.
Immediately pour the mixture into the prepared tin and allow to cool. When almost set, mark the fudge into squares with an oiled knife. Cut when completely cold and firm and store in an airtight tin, interleaved with waxed paper or baking paper. The fudge will keep for up to four weeks.