Combine the peanut butter and melted butter in a microwave-safe bowl. Microwave in 30-second bursts, stirring each time, until the mixture is smooth and fully melted, about 1.5 minutes total. The aroma of warm peanut butter and butter should fill the air.
Add the graham cracker crumbs and sifted powdered sugar to the melted mixture. Stir well with a spatula or spoon until everything is evenly combined and the mixture feels thick but still spreadable.
Press the peanut butter mixture evenly into the prepared parchment-lined pan, using the back of a spatula or your fingers to flatten the surface. Chill in the fridge for 20–30 minutes until firm and set.
Meanwhile, melt the chocolate chips in a microwave-safe bowl in short 20-second bursts, stirring well after each, until smooth and glossy. The chocolate should be silky and fragrant.
Remove the chilled peanut butter layer from the fridge. Pour the melted chocolate over the top, spreading it evenly with a spatula or the back of a spoon. The chocolate should glide smoothly over the surface.
Sprinkle a pinch of flaky sea salt over the chocolate, if using, to enhance the flavor contrast. Return the pan to the fridge for at least 30 minutes, or until the chocolate is fully set and the bars are firm.
Once set, use a sharp knife to cut the bars into neat squares. Wipe the knife with warm water between cuts for clean edges. These bars are best enjoyed chilled or at room temperature.
Serve immediately or store in an airtight container in the fridge for up to a week. For longer storage, wrap tightly and freeze for up to a month. Allow to sit at room temperature for 10 minutes before serving for the perfect chewy bite.
