Line your baking dish with parchment paper and set aside. In a mixing bowl, combine crushed graham crackers with melted butter and stir until evenly coated, resembling wet sand.
Press the crumb mixture firmly into the bottom of the prepared dish, creating an even, compact crust. Use the back of a spatula or your fingers to smooth it out thoroughly. Chill in the fridge for 15 minutes while you prepare the caramel.
In a small saucepan over medium-low heat, combine sweetened condensed milk, golden syrup, and butter. Stir constantly as the mixture begins to bubble gently after about 5 minutes, releasing a warm, sweet aroma. Continue stirring until the caramel thickens and turns a rich golden color, about 8-10 minutes.
Remove the caramel from heat and stir in vanilla extract and a pinch of sea salt. Pour the hot caramel over the chilled crust, spreading it out with a spatula to form an even layer. It should be glossy and slightly thick but pourable.
While the caramel cools slightly, melt the dark chocolate in a microwave or double boiler until smooth and glossy. Drizzle the melted chocolate over the caramel layer, creating a decorative swirl pattern.
Refrigerate the assembled bars uncovered for at least 2 hours, or until the caramel is firm and set. For quicker results, freeze for 1 hour. Once chilled, lift the bars out using the parchment paper and cut into squares with a sharp knife.
Serve the bars chilled, enjoying the crisp base contrasted with the sticky, rich caramel and glossy chocolate topping. Store leftovers in an airtight container in the fridge for up to 4 days.
