Step 1: Prepare the Pan

1. Line an 8×8-inch baking pan with parchment paper, leaving enough overhang on the sides to lift the fudge out easily.
2. Lightly grease the parchment paper with nonstick spray or butter to prevent sticking.

Step 2: Melt the White Chocolate Base

1. In a medium saucepan, combine the white chocolate chips and sweetened condensed milk.
2. Place the saucepan over low heat, stirring constantly until the white chocolate is melted and the mixture is smooth.
3. Remove from heat and stir in the vanilla extract.

Step 3: Create the Chocolate Swirl

1. In a microwave-safe bowl, combine the semi-sweet chocolate chips and heavy cream.
2. Microwave in 20-second intervals, stirring between each, until the chocolate is fully melted and smooth.

Step 4: Assemble the Fudge

1. Pour half of the white chocolate mixture into the prepared pan and spread it evenly with a spatula.
2. Sprinkle half of the chopped peanut butter cups over the white chocolate layer.
3. Pour the remaining white chocolate mixture on top, spreading it evenly.
4. Drizzle the melted chocolate over the top of the fudge and use a knife or toothpick to swirl it gently, creating a marbled effect.

Step 5: Add Toppings

1. Sprinkle the remaining peanut butter cups over the top of the fudge, pressing them gently into the surface.
2. Add additional whole peanut butter cups if desired for a decorative finish.

Step 6: Chill and Set

1. Refrigerate the fudge for at least 3 hours, or until completely firm.
2. Once set, use the parchment paper overhang to lift the fudge out of the pan.

Step 7: Slice and Serve

1. Cut the fudge into small squares using a sharp knife. For cleaner slices, warm the knife under hot water and wipe it dry between cuts.
2. Serve immediately or store for later.