Line an 8x8-inch pan with parchment or foil, leaving an overhang on two sides for easy removal. Lightly coat with cooking spray or butter.
In a medium microwave-safe bowl, combine the peanut butter and half of the sweetened condensed milk. Microwave on high in 30-second intervals, stirring after each, until the mixture is smooth and slightly thick, 60-90 seconds total. Set aside.
In another microwave-safe bowl, combine the chocolate chips, vanilla, salt, and the remaining sweetened condensed milk. Microwave on high in 30-second intervals, stirring after each, until the chocolate is just melted and the mixture is very smooth, 60-90 seconds total. If a few small chips remain, let the bowl stand for 1-2 minutes and stir again until fully melted.
Pour the chocolate mixture into the prepared pan and spread it into an even layer. Spoon dollops of the peanut butter mixture over the top. Using a butter knife or skewer lightly coated with cooking spray, gently swirl the two mixtures together to create a marbled effect without fully blending them. If using, sprinkle chopped peanuts evenly over the top and press them very lightly so they adhere.
Cover the pan and refrigerate until firm, 3-4 hours.
Lift the fudge out of the pan using the parchment or foil overhang, transfer to a cutting board, and cut into small squares. Serve chilled or at cool room temperature.