Line an 8-inch square baking dish with parchment paper, leaving some overhang for easy removal later.
In a medium saucepan over low heat, combine the chocolate chips, sweetened condensed milk, and butter, stirring continuously until the mixture is smooth and fully melted.
Remove the saucepan from heat and stir in the vanilla extract and salt until well combined.
Fold in the coarsely chopped walnuts, ensuring they are evenly distributed throughout the chocolate mixture.
Pour the fudge mixture into the prepared baking dish, spreading it out evenly with a spatula.
If desired, sprinkle flaky sea salt on top for an added burst of flavor.
Refrigerate the fudge for at least 4 hours, or until it is firm and set.
Once set, lift the fudge out of the baking dish using the parchment paper overhang and cut into squares.
Notes
For best results, allow the fudge to set in the refrigerator for at least 4 hours.