01 - Line an 8-inch square 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 2 tbsp butter. Stir constantly until smooth and melted. Remove from heat, stir in 1 tsp vanilla extract.
03 - Pour the chocolate mixture into the prepared pan and spread evenly.
04 - In a medium bowl, mix peanut butter, powdered sugar, 2 tbsp softened butter, and ½ tsp vanilla extract until smooth and well combined.
05 - Drop spoonfuls of the peanut butter mixture over the chocolate layer. Gently spread to cover the chocolate evenly.
06 - Swirl the top with a knife or skewer for a marbled effect, if desired.
07 - Refrigerate for at least 2 hours, or until set.
08 - Lift the fudge out of the pan using the parchment overhang. Cut into 24 squares. Serve chilled or at room temperature.