Step 1: Prepare Your Pan
Line a 9×13-inch baking pan with parchment paper, leaving an overhang on two sides to create handles for easy removal later. Alternatively, you can use aluminum foil lightly greased with butter or cooking spray.

Step 2: Make the Peanut Butter Base
In a large mixing bowl, combine the melted butter and graham cracker crumbs. Stir until the crumbs are fully moistened and the mixture looks like wet sand. Add the peanut butter and powdered sugar, mixing thoroughly until everything is completely combined and smooth. The mixture will be thick and slightly crumbly but should hold together when pressed. Add vanilla extract and salt if using.

Step 3: Press Into Pan
Transfer the peanut butter mixture to your prepared pan. Using your hands or the back of a spatula, press the mixture firmly and evenly into the pan, making sure to get into all corners. Apply good pressure to compact the layer—this ensures clean cuts later. The surface should be as smooth and level as possible.

Step 4: Melt the Chocolate
In a microwave-safe bowl, combine the chocolate chips and 2 tablespoons of peanut butter. Microwave in 30-second intervals, stirring between each interval, until completely melted and smooth (usually takes 1-2 minutes total). Be careful not to overheat, as chocolate can seize if it gets too hot. Alternatively, melt using a double boiler on the stovetop.

Step 5: Add the Chocolate Layer
Pour the melted chocolate mixture over the peanut butter layer. Use an offset spatula or the back of a spoon to spread it evenly across the entire surface, working quickly before the chocolate begins to set. If desired, sprinkle flaky sea salt over the top while the chocolate is still wet.

Step 6: Chill Until Set
Refrigerate the pan for at least 2 hours, or until the chocolate is completely firm and set. For faster results, you can place them in the freezer for about 45 minutes, though refrigerating gives a better texture.

Step 7: Cut and Serve
Once fully chilled, use the parchment paper overhang to lift the entire slab out of the pan and onto a cutting board. Using a sharp knife (wipe it clean between cuts for neat edges), slice into 24 squares or your desired size. For the cleanest cuts, let the bars sit at room temperature for about 5 minutes before slicing.