Line your square pan with parchment paper, leaving some overhang for easy removal.
Break the biscuits into uneven small chunks, about 1-2 cm, to add texture and variety to the bars.
Place the chopped dark chocolate into a heatproof bowl.
Set the bowl over a pot of simmering water (about 80°C/176°F) and stir gently until the chocolate is silky and smooth, about 3-4 minutes.
Add the butter cubes to the melted chocolate and continue stirring until fully melted and glossy, about 1-2 minutes.
Pour the glossy chocolate-butter mixture over the broken biscuits in a large bowl.
Stir gently with a spatula until all biscuit pieces are coated and sticky with the chocolate mixture.
Transfer the mixture into your prepared pan and press down firmly with the back of the spatula to create an even layer.
Sprinkle a tiny pinch of sea salt on top for a flavor contrast and optional extra crunch.
Place the pan in the fridge and chill for at least 2 hours, or until the mixture is firm and shiny on top.
Once set, lift the block out of the pan using the parchment overhang and cut into squares with a sharp knife.
Serve immediately or store in an airtight container in the fridge for up to a week. Enjoy these rich, crunchy, and glossy chocolate biscuit bars!