Line your baking dish with parchment paper, leaving some overhang for easy removal.
Chop the dark chocolate into small pieces so it melts quickly and smoothly.
Place the chopped chocolate and butter into a heatproof bowl.
Gently melt the mixture over a pot of simmering water, stirring frequently, until the mixture is glossy and completely smooth.
Remove the bowl from heat and stir in the sweetened condensed milk and vanilla extract until fully combined and glossy.
Crush the digestive biscuits into small chunks or crumbs, then fold them into the chocolate mixture until evenly coated.
Pour the mixture into the prepared dish, then use the spatula to spread it evenly and press down gently to compact.
Sprinkle sea salt flakes or other toppings over the surface and press lightly to embed them.
Place the dish in the fridge and chill for at least 3 hours or overnight until set and firm.
Once set, lift the slice out of the dish using the overhanging parchment paper. Slice into squares with a sharp knife.
Serve immediately or store in an airtight container in the fridge for up to 5 days.