Line your baking dish with parchment paper, leaving overhang on the sides for easy removal.
Place the chopped dark chocolate and butter in a saucepan over very low heat. Stir gently and constantly until both are melted and the mixture is smooth and glossy, filling your kitchen with a rich chocolate aroma.
Remove the saucepan from heat and stir in the sweetened condensed milk and vanilla extract until fully combined, making the mixture thick and shiny.
Gently fold in the crushed digestive biscuits, ensuring they’re evenly coated with the chocolate mixture. The texture should start to feel sticky and crumbly.
Pour the mixture into your prepared dish, spreading it out evenly with a spatula. Press down gently to settle and smooth the surface.
Sprinkle a pinch of sea salt on top, if using, for that subtle salty contrast, and add a few flakes of flaky sea salt for a finishing touch.
Refrigerate the slice for at least 3 hours, or preferably overnight, until it’s firm and crackly on top.
Once set, lift the slice out of the dish using the parchment overhang. Place it on a cutting board and, with a sharp knife, cut into squares or rectangles.
Serve immediately, or store in an airtight container in the fridge for up to a week. Enjoy the rich, crumbly texture with a cup of tea or glass of milk.
