Line your 8-inch square pan with parchment paper, leaving some overhang on the sides for easy lifting.
In a microwave-safe bowl, melt the butter and peanut butter together in short 20-30 second bursts, stirring after each until the mixture is smooth and fragrant.
Add the graham cracker crumbs and powdered sugar to the melted mixture, then stir until everything is well combined and the mixture feels thick and slightly sticky.
Press this peanut butter mixture firmly into the prepared pan, spreading evenly with the back of a spoon or your fingers to create a flat, compact layer.
In a separate microwave-safe bowl, melt the chocolate chips with the butter in 20-30 second intervals, stirring well until the mixture is glossy and smooth.
Pour the melted chocolate over the peanut butter layer, spreading it evenly with a spatula to cover completely and create a shiny top.
Place the pan in the fridge and chill for at least 2 hours, or until the chocolate is fully set and the bars are firm to the touch.
Once chilled, lift the bars out of the pan using the parchment overhang and place on a cutting board.
Use a sharp knife warmed under hot water to cut the bars into clean squares, wiping the blade between cuts for neat slices.
Serve immediately or store in an airtight container in the fridge for up to a week, letting them sit at room temperature for 10 minutes before enjoying for the best texture.