Line an 8-inch square baking pan with parchment paper, leaving some overhang for easy removal later.
In a medium saucepan over low heat, combine the chocolate chips and sweetened condensed milk. Stir continuously until smooth and melted.
Remove from heat and stir in the vanilla extract and salt. Pour half of the chocolate mixture into the prepared baking pan and spread it evenly.
In a separate bowl, heat the peanut butter in the microwave for about 20-30 seconds until it's slightly warm and easy to blend. Mix in the remaining half of the chocolate mixture.
Drop spoonfuls of the peanut butter-chocolate mixture on top of the chocolate layer in the pan.
Using a skewer or knife, gently swirl the two mixtures together to create a marbled effect, being careful not to fully mix them.
If desired, sprinkle chopped nuts on top for added crunch.
Refrigerate the fudge for at least 2 hours or until firm.
Once set, lift the fudge out using the parchment overhang and cut it into squares.
Notes
Serve the fudge on a decorative plate, and garnish with additional chopped nuts or a drizzle of melted chocolate for an elegant touch.