Line your baking dish with parchment paper for easy removal.
Place the chopped dark chocolate into a heatproof bowl.
Gently melt the chocolate in the microwave in 30-second bursts, stirring each time until smooth and glossy.
Add the butter to the melted chocolate and stir until fully incorporated, creating a shiny, smooth mixture.
Pour in the sweetened condensed milk and stir well, until the mixture is thick, sticky, and uniform in texture.
Fold in the crushed biscuits, along with any nuts or dried fruits, ensuring they are evenly distributed throughout the mixture.
Transfer the mixture into your prepared dish and use a spatula to press it down firmly, smoothing the surface for an even layer.
Place the dish in the fridge and chill for at least 2 hours, or until the slice is firm and set.
Once set, remove the slice from the fridge and lift it out of the dish using the parchment paper. Use a sharp knife to cut into squares or rectangles.
Serve immediately or keep refrigerated until ready to enjoy. The slice will be glossy, firm, and packed with chocolate flavor and crunch.