01 - Line an 8x8-inch baking pan with parchment paper, allowing an overhang on the sides to facilitate easy removal of the fudge later.
02 - Combine the chocolate chips, sweetened condensed milk, and 2 tablespoons of butter in a medium saucepan. Cook over low heat, stirring constantly until the mixture is completely melted and smooth. Remove from the heat and stir in the vanilla extract.
03 - In a separate bowl, whisk together the creamy peanut butter, melted butter, and powdered sugar until the mixture is smooth and creamy.
04 - Pour the melted chocolate mixture into the prepared pan and spread it into an even layer. Drop spoonfuls of the peanut butter mixture over the top of the chocolate layer.
05 - Use a knife or a skewer to gently swirl the peanut butter into the chocolate, creating a marbled pattern without overmixing.
06 - Refrigerate the pan for at least 2 hours, or until the fudge is firm and completely set.
07 - Lift the fudge out of the pan using the parchment paper overhang. Cut into 16 squares and serve immediately or store for later.