Preheat oven to 375°F (190°C). Butter a 12” round or 9x11” baking dish (at least 2” deep).
In a double boiler or heatproof bowl over a simmering pot, combine heavy cream, espresso powder, chocolate chips, and vanilla. Stir until fully melted and smooth.
Pour into the base of the prepared baking dish.
In a large bowl, whisk together flour, graham cracker crumbs, sugar, baking powder, salt, and cinnamon.
Cut in the butter using a pastry cutter or your fingers until mixture resembles pea-sized crumbs.
Stir in vanilla and buttermilk until just combined (do not overmix).
Scoop cobbler topping over the chocolate layer using a spoon or scoop, leaving space between for expansion.
Tuck half the marshmallows in between the cobbler topping.
Bake for 35–40 minutes, or until cobbler topping is golden brown.
Remove, top with remaining marshmallows, and bake for an additional 2–3 minutes until marshmallows are golden and gooey.
Let cobbler rest for 10 minutes to allow the chocolate to thicken. Serve warm and enjoy!
