Step 1: Line an 8×8 inch (20×20 cm) square baking tin with parchment paper, leaving an overhang on the sides for easy removal.
Step 2: In a medium bowl, combine the crushed biscuits and 150g of melted butter. Mix until the crumbs are evenly coated.
Step 3: Press the biscuit mixture firmly into the bottom of the prepared tin. Use the back of a spoon or a flat-bottomed glass to ensure it is tightly packed. Place in the freezer for 10-15 minutes.
Step 4: In a heavy-bottomed saucepan, combine the remaining 150g of butter, brown sugar, and the sweetened condensed milk.
Step 5: Heat over medium-low heat, stirring constantly, until the butter is melted and the sugar has dissolved.
Step 6: Turn the heat up to medium and bring the mixture to a gentle boil. Continue to stir constantly for 5-8 minutes until the caramel thickens and turns a golden amber color. Do not leave it unattended as it can burn easily.
Step 7: Pour the hot caramel over the chilled biscuit base and spread it evenly with a spatula. Allow it to cool at room temperature for 20 minutes, then refrigerate for at least 1 hour until set.
Step 8: Once the caramel is firm, melt the chocolate (and oil, if using) in the microwave in 30-second increments, stirring between each, until smooth.
Step 9: Pour the melted chocolate over the caramel layer and spread it to the edges. Sprinkle with flaky sea salt if desired.
Step 10: Refrigerate for at least 2 hours, or until the chocolate is fully set. Lift the bars out using the parchment paper overhang and slice into squares with a warm knife.