2 ¼ cups graham cracker crumbs (about 17-20 crackers)
1 tablespoon firmly packed dark brown sugar
⅔ cup unsalted butter, melted
7 ½ ounces milk chocolate, coarsely chopped
7 ½ ounces dark chocolate (60 to 72% cacao), coarsely chopped
1½ teaspoons light corn syrup
1 cup heavy cream
10 marshmallows, cut into quarters
½ cup lightly salted whole peanuts
½ cup chopped lightly salted peanuts
Preheat the oven to 300 degrees F. Butter the sides and bottom of a 9-by-13- inch baking pan or spray it with nonstick cooking spray.
In a large bowl, stir together the graham cracker crumbs and brown sugar. Add the butter. Use your hands to combine the mixture, then turn it out into the prepared pan. Using your hands, press the crust into an even layer along the bottom and up the sides of the pan. Use the bottom of a measuring cup to create a perfectly even crust.
Bake the crust for 10 to 15 minutes, or until the crust is golden brown. Remove the pan from the oven and place on a cooling rack.
In a large heatproof bowl, toss the chocolates together. Drizzle the corn syrup over the chocolate and set aside.
In a medium saucepan, bring the cream just to a boil. Remove from the heat and pour the cream over the chocolate mixture. Let stand for 2 to 3 minutes. Starting in the center of the bowl and working your way out to the edges, whisk the chocolate mixture in a circle until completely smooth. Fold in the marshmallows and the whole peanuts. Pour the mixture into the prepared pan and use an offset spatula to spread it as evenly as possible. Sprinkle the top with the chopped peanuts.
Refrigerate for at least 3 hours, or until set. Cut into squares and serve. The bars will keep in the refrigerator, tightly covered, for up to 3 days.