01 - Line an 8-inch square baking pan with parchment paper, allowing 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 continuously until completely melted and smooth, then remove from heat.
03 - Stir 1 tsp vanilla extract into the melted chocolate mixture until fully incorporated.
04 - Pour the chocolate mixture into the prepared pan and spread into an even layer using a spatula.
05 - In a medium bowl, combine creamy peanut butter, powdered sugar, softened butter, and ½ tsp vanilla extract. Mix until smooth and thoroughly blended.
06 - Drop spoonfuls of the peanut butter mixture over the chocolate layer, then gently spread to cover the chocolate completely.
07 - Swirl the top with a knife or skewer in a figure-eight motion to create a marbled effect, if desired.
08 - Refrigerate for at least 2 hours until completely firm and set.
09 - Lift the fudge from the pan using the parchment overhang. Cut into 24 equal squares. Serve chilled or allow to come to room temperature before serving.