Line a 8 x-8 inch baking dish with parchment paper, allowing extra parchment to hang over the edges.
Mix graham cracker crumbs and melted butter until well blended. Press this mixture into the bottom of the prepared baking dish. Place in freezer while you make the filling.
Place the chopped chocolate into a heat-proof bowl. In a saucepan, heat 2/3 cup heavy cream and espresso powder over medium-high heat until it only just begins to boil.
Pour the hot heavy cream over the chocolate. Allow to stand for 3 - 4 minutes, then stir until you get a glossy, smooth chocolate ganache. Stir in salt. Let cool until warm.
In a separate bowl, whip the remaining 1 cup heavy cream until stiff peaks only just begin to form.
Once the chocolate ganache has cooled, gently fold the whipped cream into it until fully incorporated.
Transfer the chocolate filling into the graham cracker crust and smooth out the top. Refrigerate for at least 4 hours, or until the filling is completely set.
Slice into squares. Place a marshmallow on top of each bar. Toast the marshmallows with a kitchen torch. Serve!