Line your baking dish with parchment paper, leaving some overhang for easy lifting, and set aside.
Break the biscuits into uneven small pieces, aiming for a mix of fine crumbs and larger chunks for added crunch.
In a small saucepan, gently melt the butter over low heat until just melted, about 2 minutes, and let the aroma develop as it becomes glossy.
Add the chopped dark chocolate to the melted butter and stir gently with a spatula until completely smooth and shiny, about 3-4 minutes. The mixture should feel thick and luscious.
Remove the saucepan from heat and fold in the crushed biscuits, ensuring they are evenly coated with the chocolate mixture. The mixture may look thick but sticky.
Pour the mixture into your prepared dish, spreading it out evenly with the spatula. Press down firmly to create a compact, level surface.
Sprinkle a pinch of sea salt over the top for a flavorful contrast, and gently press it into the surface.
Place the dish in the fridge and chill for at least 2 hours, or until the mixture is firm and set.
Once set, lift the bars out of the dish using the parchment overhang and transfer to a cutting board.
Use a sharp knife to cut into squares or rectangles, cleaning the blade between cuts for neat slices.
Serve immediately or store in an airtight container in the fridge for up to a week. For softer bars, let sit at room temperature for 10 minutes before enjoying.
