Prep the pan. Heat the oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment, leaving overhang for easy lifting. Lightly grease the parchment.

Make the crust. Stir graham crumbs, melted butter, brown sugar, salt, flour, and vanilla until the mixture looks like damp sand. It should hold when pressed.

Press and pre-bake. Firmly press the mixture into an even layer in the pan. Use the bottom of a measuring cup to compact. Bake 8–10 minutes, until lightly set and fragrant.

Make the chocolate layer. In a microwave-safe bowl or small pot, warm the heavy cream until steaming. Add the milk and semisweet chocolate. Let sit 1 minute, then stir until smooth and glossy. If using peanut butter, whisk it in now.

Spread the chocolate. Pour over the warm crust and spread to the edges. Let stand 5 minutes to begin setting.

Add marshmallows. Scatter mini marshmallows evenly over the top. Gently press so they stick to the chocolate.

Toast in the oven. Return the pan to the oven for 6–8 minutes, until the marshmallows are puffed and golden. For extra color, broil for 30–60 seconds, watching closely. Do not walk away.

Cool and set. Let the pan cool on a rack for at least 1 hour, then refrigerate 20–30 minutes for cleaner slices. Lift out using the parchment and cut with a lightly oiled knife.

Finish and serve. Sprinkle flaky sea salt if you like. Serve at room temp for the best texture.