Line an 8×8-inch (20×20 cm) baking pan with parchment paper, leaving overhang on two sides to form a sling. Lightly spray the parchment with nonstick spray if desired to prevent sticking.

In a medium mixing bowl, stir together the melted butter and 1 cup creamy peanut butter until smooth and glossy. Add the powdered sugar, graham cracker crumbs, vanilla, and fine sea salt. Mix with a sturdy spoon or spatula until everything is well combined and no dry patches remain. The mixture should be thick and slightly crumbly but hold together when pressed, like a soft cookie dough.

Transfer the peanut butter mixture to the prepared pan. Use clean hands or the back of a spoon to press it firmly and evenly into the bottom of the pan. Take a moment to really pack it down so the bars slice cleanly later. For an extra-smooth surface, place a piece of parchment on top and press with the bottom of a measuring cup.

In a microwave-safe bowl, combine the semi-sweet chocolate chips and 1/4 cup creamy peanut butter. Microwave in 20–30 second bursts, stirring well after each interval, until the chocolate is just melted and smooth. Do not overheat; the residual heat will finish melting the chocolate as you stir.

Pour the warm chocolate-peanut butter mixture over the peanut butter base. Gently tilt the pan and use an offset spatula or the back of a spoon to spread it into an even layer all the way to the edges. If using flaky sea salt, chopped peanuts, mini chocolate chips, or sprinkles, add them now while the chocolate is still soft so they adhere.

Refrigerate the pan for at least 2 hours, or until the chocolate is set and the bars feel firm when you gently press the center. For faster setting, you can chill in the freezer for about 45 minutes, though the texture is slightly better with a slower chill in the refrigerator.

Once fully set, use the parchment sling to lift the slab out of the pan and place it on a cutting board. Let sit at room temperature for 5–10 minutes to slightly soften the chocolate, which helps prevent cracking. Use a sharp knife, wiping it clean between cuts, to slice into 16 small squares or 12 larger bars.

Serve the bars chilled or just slightly cool. Store leftovers in an airtight container in the refrigerator for up to 1 week, layering with parchment or wax paper so the bars don’t stick together. For longer storage, freeze for 2–3 months.