Preparation of Baking Pan: Begin by greasing an 8-inch square baking pan and lining it with either baking or parchment paper to facilitate easy removal of the fudge once set.
Combining Base Ingredients: In a medium-sized saucepan, combine the sweetened condensed milk, brown sugar, unsalted butter, and golden syrup. Place this mixture over a low heat setting, stirring continuously until the ingredients are fully combined and the sugar has completely dissolved.
Cooking the Fudge Mixture: Increase the heat to a medium setting. Continue to stir the mixture consistently. Maintain this process until the mixture begins to simmer and noticeably thickens. This stage may take approximately 5-6 minutes. It is crucial to avoid overheating or burning the mixture during this process.
Incorporating White Chocolate: Once the caramel mixture has thickened, remove the saucepan from the heat. Immediately stir in the white chocolate pieces until they are completely melted and uniformly incorporated into the mixture.
Setting the Fudge: Pour the fudge mixture into the prepared baking pan, ensuring a smooth and even surface. Sprinkle sea salt evenly over the top of the mixture. Refrigerate the pan for 3-4 hours, or preferably overnight, to allow the fudge to set fully.
Serving the Fudge: Once set, remove the fudge from the refrigerator. Using a sharp knife, cut the fudge into evenly sized squares. These squares are now ready to be served.