1. Line an 8x8-inch square pan with foil, leaving enough to hang over the sides, then butter the foil. Set aside.
2. In a medium saucepan, melt the ½ cup of butter over low heat. Once melted, remove from heat and stir in the powdered sugar until smooth. Stir in the peanut butter and graham cracker crumbs until completely combined and smooth. Spread the peanut butter mixture into the prepared pan and smooth the top.
3. In the now-empty saucepan, melt the ¼ cup unsalted butter. Add the chocolate and stir until almost completely melted. Remove from heat and continue stirring until completely melted. Pour the melted chocolate over the peanut butter layer and smooth into an even layer.
4. Refrigerate for at least 30 minutes to set. Remove from the pan using the foil overhangs and place on a cutting board. Use a pizza cutter to slice into 16 bars. Store in an airtight container in the refrigerator.