Prep the pan. Line a 9x13-inch baking pan with parchment, leaving overhang on two sides for easy lifting.
Lightly butter or spray the parchment.
Mix the base. In a large bowl, stir together melted butter, 2 cups peanut butter, graham cracker crumbs, powdered sugar, vanilla, and salt. The mixture should look thick, glossy, and a bit like cookie dough. If it seems dry, add 1–2 tablespoons more melted butter; if too soft, add a few more crumbs.
Press and smooth. Transfer the peanut butter mixture to the pan.
Press firmly into an even layer. Use the back of a measuring cup to smooth the top and compact the base so it holds together well.
Melt the chocolate. In a microwave-safe bowl, combine chocolate chips and 2 tablespoons peanut butter. Microwave in 20–30 second bursts, stirring between each, until smooth and glossy.
You can also melt in a heatproof bowl set over a pot of barely simmering water.
Top the bars. Pour the warm chocolate mixture over the peanut butter layer. Tilt the pan to spread, or use an offset spatula to smooth to the edges. Tap the pan gently to remove any air bubbles.
Chill to set. Refrigerate uncovered for 1–2 hours, or until the chocolate is firm to the touch.
For a cleaner slice, wait the full 2 hours.
Slice and serve. Lift the slab out using the parchment. Warm a knife under hot water, wipe it dry, and cut into bars or squares. Sprinkle flaky sea salt on top if you like a sweet-salty finish.