Line a 9x13-inch (23x33 cm) baking pan with parchment paper, leaving an overhang on two opposite sides. This will make it easier to lift the fudge out later.
In a large saucepan over low heat, combine the chocolate chips, sweetened condensed milk, and butter. Stir continuously until the mixture is smooth and well-combined. Be careful not to burn the chocolate.
Once the mixture is fully melted, remove the saucepan from the heat. Stir in the vanilla extract and, if desired, fold in the chopped nuts until evenly distributed.
Pour the fudge mixture into the prepared pan, spreading it evenly with a spatula.
Allow the fudge to cool at room temperature for about 2 hours. Once it reaches room temperature, transfer it to the refrigerator and let it set for an additional 3-4 hours, or until firm.
Once the fudge is completely set, use the parchment paper overhangs to lift it out of the pan. Place it on a cutting board and use a sharp knife to cut it into squares.
Arrange the fudge squares on a serving platter and enjoy this classic chocolate treat!