01 - Line an 8x8-inch baking pan with parchment paper, leaving an overhang for easy removal.
02 - In a medium saucepan over low heat, combine chocolate chips, sweetened condensed milk, and butter. Stir constantly until smooth and melted. Remove from heat and whisk in vanilla extract.
03 - In a separate bowl, blend peanut butter, melted butter, and powdered sugar until smooth.
04 - Transfer the chocolate mixture into the prepared pan and spread evenly.
05 - Drop spoonfuls of the peanut butter mixture onto the chocolate layer. Use a knife or skewer to gently swirl and create a marbled pattern.
06 - Lightly tap the pan on the countertop to release any air bubbles.
07 - Refrigerate for at least 2 hours until firm.
08 - Lift the set fudge from the pan using the parchment overhang. Cut into 36 squares. Serve chilled or at room temperature.