Line your 8-inch square pan with parchment paper, leaving overhang for easy removal.
In a microwave-safe bowl, combine the peanut butter and butter. Microwave in 20-second bursts, stirring each time until the mixture is smooth, fragrant, and slightly bubbly—about 1 minute total.
Add the sweetened condensed milk to the melted peanut butter mixture. Stir until fully blended and glossy—this sticky mixture will be thick and fragrant.
In a separate bowl, crush the graham cracker crumbs into a fine powder. Fold the crumbs into the peanut butter mixture, stirring gently until evenly incorporated and thick.
Transfer the mixture into the prepared pan. Use a spatula or the back of a spoon to press it evenly into the corners and smooth the surface. Press firmly to compact the layer.
In a microwave-safe bowl, melt the chocolate chips in 15-second bursts, stirring until smooth and shiny. If needed, add a tiny splash of coconut oil to achieve a glossy, smooth consistency.
Pour the melted chocolate over the peanut butter layer, spreading it evenly with a spatula. Tap the pan lightly on the counter to settle the chocolate and remove air bubbles.
Place the pan in the fridge for at least 2 hours, or until the chocolate is fully firm and the bars feel set when gently pressed.
Once chilled, lift the bars out of the pan using the parchment overhang. Use a sharp knife to cut into 16 equal squares, wiping the knife clean between cuts for neat edges.
Let the sliced bars sit at room temperature for 10 minutes before serving, so the chocolate softens slightly and the flavors meld. Enjoy these rich, chewy, and crunchy bars!