Line a pan with foil and spray with nonstick cooking spray.
Heat the chocolate on a double boiler on the stovetop. Heat the condensed milk, vanilla and salt on the stovetop in a heavy-bottomed pan over medium heat and stir until smooth. Add the melted chocolate and combine –the mixture will be thick and hard to stir. Add the chopped nuts and/or other mix-ins and pour into the lined pan.
Allow the fudge to cool in the refrigerator, uncovered, for 3 hours or overnight.