For the crust:
1/2 cup unsalted butter, melted
1/4 cup light brown sugar
1 1/4 cups graham cracker crumbs (about 10 sheets crushed)
1/4 teaspoon salt
For the filling:
2 3/4 cups heavy whipping cream, divided
2 cups milk chocolate chips
1 tablespoon unsweetened cocoa powder
1/2 teaspoon vanilla extract
1/8 teaspoon kosher salt
For the topping:
1 cup large marshmallows
1 cup mini marshmallows
2 tablespoons chocolate chips
2 tablespoons graham cracker crumbs
1 tablespoon chocolate syrup
To make crust: In a large bowl, combine butter, sugar, graham cracker crumbs, and salt and mix until the texture resembles wet sand. Press the graham cracker mixture into a 9-inch pie pan and refrigerate for about 30 minutes until firm.
To make filling: Meanwhile, bring 3/4 cup heavy cream to a simmer over low heat and set aside. In a large bowl, combine chocolate chips, cocoa powder, vanilla extract, and salt. Pour the heavy whipping cream over the chocolate and let sit for 2 to 3 minutes until the chocolate begins to melt. Whisk the cream and chocolate together until it becomes silky and shiny. Place the bowl in the refrigerator and chill for 20 minutes.
In a large mixing bowl, using a hand mixer, whip the remaining heavy whipping cream until stiff peaks form. Gently fold the cooled chocolate into the whipped cream mixture until fully incorporated. Be careful to not overmix.
To assemble: Transfer the chocolate mousse into the chilled graham cracker crust, cover with plastic wrap, and refrigerate for 3 hours until firm.
Just before serving, top the pie with marshmallows and use a minitorch to toast them until golden brown. Top with chocolate chips and graham cracker crumbs, and drizzle chocolate syrup over everything.