Line your 20cm (8-inch) pan with parchment paper, leaving overhang on the sides for easy removal.
In a microwave-safe bowl, combine the melted butter and peanut butter. Microwave in 30-second bursts, stirring until smooth and glossy with a fragrant nutty aroma.
Add the sifted powdered sugar and graham cracker crumbs to the peanut butter mixture. Mix thoroughly with a spatula until the mixture is thick, sticky, and well combined—this will take about a minute and should look slightly glossy and cohesive.
Press the peanut butter mixture evenly into the prepared pan, using the back of a spatula or your fingers to smooth the top and create a firm, flat surface. You should hear a satisfying resistance as you press, and the layer will feel dense and sticky.
In a separate bowl, melt the chocolate chips and coconut oil together using short bursts in the microwave, stirring well after each to achieve a shiny, smooth chocolate sauce with a rich aroma. This should take about 1 minute in total.
Pour the melted chocolate over the peanut butter layer, spreading it evenly with a spatula or the back of a spoon. You want a smooth, glossy surface that crackles slightly as you spread, signaling a perfect coating.
Place the pan in the fridge and chill for at least 2 hours, or until the chocolate is firm and the bars are set. For quicker results, freeze for about 1 hour.
Once chilled and firm, lift the bars out of the pan using the parchment overhang. Use a sharp knife to cut into 12 even squares, pressing down firmly for clean edges.
If desired, melt a little extra chocolate and drizzle over the tops of the bars for an extra decorative touch. Allow the drizzle to set slightly before serving.
Enjoy these rich, crunchy, and decadent bars now or store them in an airtight container in the fridge for up to a week. Bring to room temperature briefly before serving for the best texture and flavor.