Line an 8-by-8-inch (20 x 20 cm) baking dish with parchment paper, allowing extra parchment to hang over the edges.
Crust: Crack graham crackers in half and place in food processor. Process until broken down into crumbs. Transfer to a bowl and stir in sugar and salt. Stir in melted butter. Press mixture into the baking dish in an even layer. Place in freezer while you make the filling.
Filling: Place ⅔ cup (160 ml) heavy cream, instant espresso powder, and McCormick Vanilla Extract in a small pot over low heat. Heat until hot, but not yet simmering. Pour hot liquid over chopped chocolate and stir until melted and smooth. Stir in salt. Let mixture cool completely.
Place the rest of the heavy cream in a large bowl. Beat with a hand mixer until you have stiff peaks. Fold in cooled chocolate mixture.
Pour filling over crust. Refrigerate for at least 4 hours.
Slice into squares. Place a marshmallow on top of each bar. Brown marshmallows with a kitchen torch or place under the broiler for 10 seconds to brown. Serve!