Line your baking pan with parchment paper to make removal easier, and set aside.
Break the dark chocolate into small, even pieces to ensure quick, uniform melting.
Place the chocolate and butter in a heatproof bowl, then melt gently over a double boiler or in short bursts in the microwave, stirring each time, until smooth and glossy.
Stir in the sweetened condensed milk and vanilla extract until the mixture is thick, shiny, and well combined, filling the kitchen with a rich chocolate aroma.
Pour the luscious chocolate mixture into your prepared pan, then use a spatula to spread it evenly across the surface, tapping the pan lightly on the counter to smooth out any bubbles.
If desired, sprinkle toasted nuts or other toppings over the surface and gently press them in for better adhesion.
Transfer the pan to the fridge and chill for at least 2 hours, or until the slice is set and firm to the touch.
Once cooled and firm, lift the slice out of the pan using the parchment paper, then place on a cutting board.
Using a sharp knife, cut into squares or rectangles, wiping the blade clean between each cut for neat, precise pieces.
Serve immediately for a chewy, fudgy treat, or store in an airtight container in the fridge for up to 5 days, or freeze for longer storage.
