Line an 8 x 8-inch baking pan with aluminum foil and lightly grease with butter or cooking spray. Set aside.
In a microwave safe bowl, combine half the sweetened condensed milk (7 oz) chocolate chips and 1 tablespoon of butter. In another microwave safe bowl, combine remaining sweetened condensed milk (7 oz), peanut butter chips and remaining tablespoon of butter.
Place both bowls in the microwave and cook on high heat in 30 second intervals, stirring in between, until chips are melted and mixtures are smooth.
Stir 1 tsp of vanilla extract and 4 oz cream cheese into each bowl until melted and smooth. Place both bowls in the microwave for 30 seconds on high heat. Stir until smooth and creamy.
Pour two-thirds of the chocolate mixture into prepared pan. Spoon half of peanut butter mixture on top of the chocolate mixture. Spoon over remaining chocolate mixture, and finally, spoon remaining peanut butter mixture on top.
Drag a knife through the fudge to create swirls. Refrigerate for 3 hours before serving, and store in a sealed container in the refrigerator.