01 - Line a 9x9 inch baking pan with parchment paper, leaving enough overhang on the sides to lift the bars out later.
02 - In a large mixing bowl, combine the melted butter, creamy peanut butter, powdered sugar, graham cracker crumbs, and vanilla extract. Stir until all ingredients are thoroughly blended and the mixture holds together when pressed.
03 - Transfer the peanut butter mixture into the prepared pan and press it down firmly into an even layer, smoothing the surface with a spatula.
04 - In a microwave-safe bowl, combine the semi-sweet chocolate chips and peanut butter. Microwave in 30-second intervals, stirring after each burst, until completely melted and glossy smooth.
05 - Pour the melted chocolate-peanut butter mixture over the pressed base and spread it into an even layer using a spatula.
06 - Refrigerate the pan for at least 1 hour, or until the chocolate topping is firm to the touch and fully set.
07 - Remove the slab from the pan using the parchment overhang and cut into 16 even squares. Store any leftovers in an airtight container in the refrigerator.