Step 1: Line an 8x8 inch square baking pan with parchment paper, leaving an overhang on the sides for easy removal.
Step 2: In a large bowl, combine the finely crushed biscuits and the 150g of melted butter. Mix until the crumbs are evenly coated.
Step 3: Press the biscuit mixture firmly and evenly into the bottom of the prepared pan. Use a flat-bottomed glass to pack it down tightly. Chill in the freezer for 15 minutes.
Step 4: In a medium saucepan, combine the 150g butter, light brown sugar, sweetened condensed milk, and golden syrup over medium-low heat.
Step 5: Stir constantly as the butter and sugar melt. Bring the mixture to a gentle boil and continue to cook for 5-7 minutes, or until the caramel has thickened and turned golden. Do not leave it unattended!
Step 6: Pour the hot caramel over the chilled biscuit base. Spread it evenly with a spatula. Place the pan in the refrigerator for at least 30-45 minutes until the caramel is firm.
Step 7: In a microwave-safe bowl, combine the chopped chocolate and oil. Heat in 20-second increments, stirring in between, until completely smooth.
Step 8: Pour the melted chocolate over the set caramel layer. Tilt the pan to ensure the chocolate reaches every corner.
Step 9: If desired, sprinkle flaky sea salt over the top. Chill in the refrigerator for at least 2 hours (or overnight) until fully set.
Step 10: Use the parchment paper to lift the bars out of the pan. Slice into 16 squares using a sharp knife dipped in hot water for clean cuts.