Line an 8x8-inch (20x20 cm) square baking pan with parchment paper, leaving some overhang on the sides for easy lifting. Lightly spritz with nonstick spray if desired.

In a large bowl, combine the melted butter, 1 cup creamy peanut butter, vanilla extract, and fine sea salt. Stir until smooth and creamy. Add the graham cracker crumbs and powdered sugar, then mix until evenly combined and a soft, thick dough-like mixture forms.

Check the consistency of the base. It should hold together when pressed. If it seems too crumbly, add 1–2 tablespoons of extra peanut butter. If it feels too soft or greasy, sprinkle in another tablespoon or two of graham cracker crumbs.

Transfer the peanut butter mixture to the prepared pan. Use a spatula or the bottom of a measuring cup to press it firmly into an even, compact layer, smoothing the top.

Place the semi-sweet chocolate chips, 1/4 cup creamy peanut butter, and butter or coconut oil (if using) in a microwave-safe bowl. Microwave in 20–30 second intervals, stirring well after each, until fully melted and smooth (about 60–90 seconds total). Alternatively, melt gently in a heatproof bowl over a pot of simmering water.

Pour the melted chocolate-peanut butter mixture over the peanut butter base. Tilt the pan and use a spatula to spread it into an even layer that reaches all the corners. Gently tap the pan on the counter to release any air bubbles.

While the chocolate is still soft, sprinkle on any garnishes you like, such as flaky sea salt, crushed peanuts, mini chocolate chips, or crushed pretzels. You can also drizzle melted white chocolate on top for contrast.

Cover the pan loosely with plastic wrap or foil and chill in the refrigerator for at least 2 hours, or until the chocolate is set and the base feels firm when pressed. For a faster set, chill in the freezer for about 45–60 minutes, noting that the chocolate will be firmer to cut.

Use the parchment overhang to lift the slab out of the pan and onto a cutting board. With a sharp knife, cut into 16 large bars or up to 25 smaller squares. For the cleanest cuts, warm the knife under hot water, dry it, and wipe the blade between cuts.

Store sliced bars in an airtight container in the refrigerator for up to 1 week, layering parchment between them. For longer storage, freeze the bars in a single layer until firm, then transfer to a freezer-safe container with parchment between layers and freeze for up to 2–3 months.