Preheat the oven to 300°F (150°C). Butter the bottom and sides of a 9×13-inch baking dish.
In a large bowl, combine the graham cracker crumbs and brown sugar. Pour in the melted butter and mix until the crumbs are evenly moistened. Press the mixture firmly into the bottom and slightly up the sides of the prepared pan. Use the bottom of a measuring cup to smooth and compact the crust.
Bake the crust for 10–15 minutes, or until lightly golden. Remove from the oven and let cool on a wire rack.
Place the milk and dark chocolate into a large heatproof bowl. Drizzle the corn syrup over the chocolate and set aside.
In a medium saucepan, heat the heavy cream just until it begins to simmer. Remove from heat and immediately pour over the chocolate. Let sit for 2–3 minutes, then whisk from the center outward until smooth and glossy.
Gently fold in the marshmallows and whole peanuts. Pour the chocolate mixture over the cooled crust and spread evenly with an offset spatula.
Sprinkle the chopped peanuts evenly over the top.
Refrigerate for at least 3 hours, or until fully set.
Slice into squares and serve.
Enjoy!
