Toll House Famous Fudge

Line an 8-inch square baking pan with foil, extending over the edges. Grease the foil with butter or cooking spray.
In a medium saucepan, combine the semi-sweet chocolate chips, sweetened condensed milk, and salt.
Cook over medium heat, stirring constantly, until the chocolate chips are melted and the mixture is smooth.
Remove from heat and stir in the vanilla extract and nuts, if using.
Pour the mixture into the prepared baking pan, spreading it evenly.
Refrigerate for at least 2 hours or until firm.
Lift the fudge out of the pan using the foil overhang and cut into squares.