Chop the dark chocolate into small pieces to ensure it melts evenly, and place it in a microwave-safe bowl.
Microwave the chocolate in 30-second bursts, stirring well after each, until it’s completely melted and glossy—this should take about 1-2 minutes.
Add the butter to the melted chocolate and stir until fully incorporated, creating a smooth, shiny mixture.
Stir in the sweetened condensed milk and vanilla extract, mixing until the mixture is thick and uniform.
Pour the chocolate mixture into your prepared square pan, spreading it evenly with a spatula and smoothing the surface for a neat finish.
If desired, sprinkle chopped nuts, dried fruit, or crispy cereal on top for added texture and flavor, pressing them lightly into the surface.
Place the pan in the fridge and chill for at least 2 hours, or until the mixture is firm and set with a glossy sheen.
Once set, remove the pan from the fridge. Use a warm, sharp knife to cut the bars into squares, wiping the blade between cuts for clean edges.
Serve the bars immediately for the best chewy, glossy finish, or store in an airtight container in the fridge for up to a week.
