Prepare the Pan: Line a 9x13 inch pan with parchment paper, leaving some overhang on the sides to act as "handles." This will make it much easier to lift the bars out for clean slicing later. For thicker bars, you can use a 9x9 inch pan.
Form the Base: In a medium bowl, combine the melted butter, graham cracker crumbs, powdered sugar, and 1 cup of peanut butter. Mix thoroughly until all the ingredients are well-blended and no dry spots of powdered sugar remain. The mixture will be thick.
Press the Base: Transfer the peanut butter mixture to your prepared pan. Using a spatula or the back of a spoon, press the mixture firmly and evenly into the bottom of the pan. A flat-bottomed glass can also help create a compact, even layer.
Prepare the Chocolate Topping: In a microwave-safe bowl, combine the milk chocolate chips and the remaining ¼ cup of peanut butter. Microwave in 30-second intervals, stirring well after each one, until the chocolate is melted and the mixture is smooth. Be careful not to overheat the chocolate.
Assemble and Chill: Pour the melted chocolate mixture over the pressed peanut butter base and spread it into an even layer with a spatula.
Set the Bars: Refrigerate the pan for at least one hour, or until the chocolate is firm. For the cleanest cuts, let the bars sit at room temperature for a few minutes before slicing so the chocolate doesn't crack.