01 - Line an 8-inch square baking pan with parchment paper, ensuring an overhang on all sides for easy removal.
02 - In a medium saucepan set over low heat, combine the semi-sweet chocolate chips, sweetened condensed milk, and 1/4 cup unsalted butter. Stir continuously until the mixture is completely melted and smooth. Remove the saucepan from the heat.
03 - Pour two-thirds of the melted chocolate mixture into the prepared pan. Using a spatula, spread it evenly across the bottom to form the initial layer. Set aside the remaining one-third of the chocolate mixture.
04 - In a separate medium bowl, combine the creamy peanut butter, powdered sugar, melted unsalted butter, and vanilla extract. Stir thoroughly until the mixture is smooth and creamy.
05 - Carefully and evenly spread the prepared peanut butter mixture over the first chocolate layer in the pan.
06 - If the reserved chocolate mixture has thickened, gently reheat it for a few seconds until it's spreadable. Carefully spread this remaining chocolate over the peanut butter layer.
07 - If desired, sprinkle the chopped roasted peanuts and flaky sea salt over the top chocolate layer for added texture and flavor.
08 - Transfer the pan to the refrigerator and chill for a minimum of 2 hours, or until the fudge is completely firm and set.
09 - Once thoroughly chilled, use the parchment paper overhang to lift the fudge from the pan. Place it on a cutting board, slice into desired squares, and serve.