Prepare the pan: Line an 8x8-inch square baking pan with parchment paper or lightly grease it.
Melt the fudge base: In a medium saucepan, melt the butter over medium heat. Add the sweetened condensed milk and chocolate chips, stirring constantly until the mixture is smooth and fully melted.
Add flavorings: Stir in the vanilla extract and a pinch of salt (if using) for extra flavor.
Swirl in the fudge sauce: Pour the melted fudge mixture into the prepared pan, spreading it evenly. Drizzle the chocolate fudge sauce over the fudge base and swirl it with a knife or spatula to create a marbled effect.
Add mix-ins: Gently fold in the chopped peanut butter cups, distributing them throughout the fudge mixture.
Chill: Refrigerate the fudge for at least 2-3 hours or until firm.
Cut and serve: Once the fudge has set, remove it from the pan and cut it into squares.