01 - Line an 8-inch square baking pan with parchment paper, leaving an overhang on opposite sides for easy removal later.
02 - Combine chocolate chips, sweetened condensed milk, and 2 tbsp butter in a medium saucepan over low heat. Stir constantly until completely smooth and melted, about 5-7 minutes. Remove from heat and stir in 1 tsp vanilla extract.
03 - Pour the melted chocolate mixture into the prepared pan. Use an offset spatula to spread it into an even layer covering the bottom of the pan completely.
04 - In a medium bowl, combine creamy peanut butter, powdered sugar, softened butter, and ½ tsp vanilla extract. Mix with a wooden spoon or hand mixer until completely smooth and well blended, about 2 minutes.
05 - Drop spoonfuls of the peanut butter mixture evenly over the chocolate layer. Gently spread to cover the chocolate completely, taking care not to mix the two layers too much.
06 - If desired, run a knife or skewer through the layers in a figure-eight or zigzag pattern to create a marbled effect. Do not over-swirl or the layers will become muddy.
07 - Refrigerate the fudge for at least 2 hours, or until completely firm and set. The top should no longer be tacky to the touch.
08 - Lift the fudge from the pan using the parchment overhang. Place on a cutting board and cut into 24 even squares, about 1½ inches each. Serve chilled or bring to room temperature for 10-15 minutes before serving for a softer texture.