Make the Base: In a medium bowl, mix crushed cookies with melted butter until well combined. Press firmly into the bottom of a parchment-lined 8×8-inch or 9×9-inch square pan. Chill in the fridge while you make the caramel.

Make the Caramel Layer: In a saucepan over medium heat, combine sweetened condensed milk, brown sugar, butter, syrup, and a pinch of salt. Stir constantly until mixture comes to a gentle boil. Reduce heat to low and continue stirring for 6–8 minutes, until thickened and golden brown. Remove from heat and stir in vanilla.

Assemble: Pour the hot caramel over the chilled shortbread base and spread evenly. Return to the fridge and chill for at least 1 hour until set.

Add the Chocolate Topping: Melt chocolate chips and heavy cream (or butter) together in the microwave in 30-second intervals, stirring between each, until smooth. Pour over the chilled caramel layer and spread evenly.

Chill and Set: Refrigerate the whole pan for at least 2 hours, or until the chocolate is firm.

Serve: Cut into squares or bars. Sprinkle with sea salt flakes if desired. Store chilled in an airtight container.