Chop the dark chocolate into small, uniform pieces and place them in a heatproof bowl.
Melt the chocolate gently in the microwave in 30-second bursts, stirring well after each, until smooth and glossy. This should take about 1-2 minutes. The chocolate will smell rich and start to shimmer as it melts.
In a mixing bowl, combine the crushed digestive biscuits with the melted chocolate. Use a spatula to fold and blend until the crumbs are evenly coated and the mixture is thick and sticky.
Add the sweetened condensed milk and vanilla extract to the mixture, folding gently until fully integrated. The mixture should become glossy and hold together when pressed.
Line your pan with parchment paper, leaving some overhang for easy removal. Spoon the mixture into the pan, then press firmly with the back of a spatula or your fingers to create an even, compact layer.
Sprinkle a tiny pinch of sea salt on top for added flavor contrast. Place the pan in the fridge and chill for at least 2 hours, or until the mixture is firm and set.
Once chilled and firm, lift the chocolate block out of the pan using the parchment overhang. Place it on a cutting board and use a sharp knife to cut into even bars or squares.
Wipe the knife with warm water between cuts for clean, smooth slices. Serve immediately or store in an airtight container in the fridge for up to a week.
