Line a 20x20cm (8x8 inch) baking dish with parchment paper, leaving some overhang for easy removal.
Place the chopped dark chocolate and butter in a heatproof bowl.
Microwave the mixture in 30-second bursts, stirring each time, until smooth and glossy, about 1.5 minutes total.
Pour in the sweetened condensed milk and stir vigorously until fully combined and shiny, creating a thick, molten fudge.
Stir in the vanilla extract and a pinch of salt, blending until fragrant and smooth.
Pour the glossy fudge mixture into the prepared dish, using a spatula to spread it evenly and tap gently to remove air bubbles.
Place the dish in the fridge and chill for at least 2 hours, or until the fudge is firm and set.
Once firm, lift the fudge out of the dish using the parchment overhang and transfer to a cutting board.
Slice into 12 even bars with a sharp knife, wiping the blade clean between cuts for neat edges.
Serve immediately or store in an airtight container in the fridge for up to a week. For softer bites, let sit at room temperature for 5 minutes before enjoying.
