Line an oven dish with parchment paper, leaving some overhang for easy removal.
Add the butter, condensed milk, light brown sugar, and golden syrup to a large saucepan.
Cook over low heat, stirring until the butter has melted and the sugar has completely dissolved.
Bring the mixture to a gentle simmer and continue cooking over low heat for 15 minutes, stirring continuously.
Test the fudge by dropping a small amount into cold water. If it forms a soft ball when rubbed between your fingers, it is ready. If not, continue cooking briefly and test again.
Pour the hot fudge into a large mixing bowl and beat for 3–5 minutes, using a wooden spoon, electric hand mixer, or food mixer, until it begins to thicken.
Quickly pour the mixture into the prepared dish and spread it evenly before it becomes too firm.
Refrigerate for at least 1 hour, then cut into approximately 25 equal squares.
