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. Press it down firmly and evenly using your hands or the back of a spatula, smoothing the surface.
04 - In a microwave-safe bowl, combine the semi-sweet chocolate chips and creamy peanut butter. Microwave in 30-second intervals, stirring after each burst, until completely melted and silky smooth.
05 - Pour the melted chocolate-peanut butter mixture over the pressed base. Spread it into an even layer using a spatula, ensuring full coverage to the edges.
06 - Refrigerate the pan for at least 1 hour, or until the chocolate topping is firm to the touch and the bars are fully set.
07 - Lift the slab out of the pan using the parchment overhang. Cut into 16 even squares. Store any leftovers in an airtight container in the refrigerator.