Put the sugar and milk into a saucepan and heat gently, stirring constantly until the sugar dissolves, about 10 minutes.
Add the sweetened condensed milk, butter, salt and golden syrup and stir until the butter has melted.
Bring to the boil and continue boiling for about 5 minutes or until it reaches the soft ball stage (114˚C on a sugar thermometer). Stir occasionally to prevent the mixture burning on the base of the pan.
Remove from the heat, add the vanilla and cool for 2–3 minutes.
Beat the fudge with a wooden spoon or electric beater for about 3 minutes until it is the consistency of thick custard. It should still level out when you remove the spoon, so check every 30 seconds or so.
Pour into the tin and mark into squares. Cut when cold.
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