Line an 8-inch square baking dish with parchment paper, leaving some overhang for easy removal.
In a medium saucepan over medium heat, combine the white chocolate chips, sweetened condensed milk, granulated sugar, and butter. Stir 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.
Pour half of the fudge mixture into the prepared baking dish, spreading it evenly.
Drizzle half of the caramel sauce over the fudge layer, then use a knife to swirl it slightly into the fudge.
Pour the remaining fudge mixture on top and spread it evenly. Drizzle the rest of the caramel sauce over the top and swirl again if desired.
If using, sprinkle the chopped nuts over the top of the fudge.
Refrigerate the fudge for at least 2 hours or until firm.
Once set, lift the fudge out of the dish using the parchment paper overhang and cut it into squares.