Begin by preparing an 8-inch square baking pan. Line it with parchment paper, ensuring that there is some overhang on the sides.
Take a medium saucepan and place it over low heat. Add the semi-sweet chocolate chips and the sweetened condensed milk. Stir the mixture continuously as it heats, until it becomes a smooth and glossy chocolate sauce.
Once the chocolate is fully melted and combined, remove the saucepan from the heat. Stir in the vanilla extract and salt until evenly mixed. Pour half of this chocolate mixture into the prepared baking pan, spreading it out evenly with a spatula.
In a separate microwave-safe bowl, add the peanut butter. Warm it in the microwave for about 20-30 seconds until it becomes slightly warm and easier to stir. Once heated, mix in the remaining half of the chocolate mixture until well combined and smooth.
Using a spoon, drop dollops of the peanut butter-chocolate mixture over the layer of chocolate in the baking pan. The spoonfuls can be placed in a random pattern for a beautiful swirl effect!
With a skewer or a butter knife, gently swirl the two mixtures together. Aim for a marbled look, but be cautious not to blend them completely.
If desired, sprinkle the chopped nuts over the top of the fudge for an extra crunch and a nice presentation.
Place the baking pan in the refrigerator and let the fudge chill for at least 2 hours, or until it has set firmly.
After the fudge is firm, carefully lift it out of the pan using the parchment overhang. Transfer it to a cutting board and cut it into squares with a sharp knife.
Notes
For an elegant touch, garnish with additional chopped nuts or drizzle melted chocolate over the squares before serving.