Line an 8x8-inch baking dish with parchment paper, allowing some overhang on the sides for easy removal later.
In a medium saucepan over low heat, combine the semi-sweet chocolate chips and sweetened condensed milk. Stir continuously until the chocolate is fully melted and the mixture is smooth.
Remove the pan from heat and stir in the vanilla extract and a pinch of salt until well mixed. Set aside.
In a separate bowl, combine the creamy peanut butter and powdered sugar. Mix until smooth.
Pour half of the melted chocolate mixture into the prepared baking dish and spread evenly.
Dollop spoonfuls of the peanut butter mixture over the chocolate layer and gently swirl it with a knife to create a marbled effect.
Pour the remaining chocolate mixture on top of the peanut butter layer and smooth it out with a spatula.
Refrigerate the fudge for at least 2 hours or until completely set.
Once set, lift the fudge out of the dish using the parchment paper overhang and cut into squares.
Notes
Make sure to let the fudge set completely before cutting for clean squares.