In a small saucepan, heat cream and corn syrup 2-3 minutes, do not let it come to a rolling boil.
Place chocolate in a large glass or metal bowl. Pour warm cream and syrup over chocolate and let set 3-4 minutes. Whisk until smooth. Set aside until ready to use.
Preheat the oven to 350º Fahrenheit.
Line 18 muffin tins with paper liners.
In a large mixing bowl, beat together the eggs, buttermilk, Greek yogurt, canola oil, and vanilla until smooth. Add the flour, sugar, unsweetened cocoa powder, baking soda, baking powder, and salt. Mix until smooth. Add the hot coffee or water and mix until combined.
Pour the batter and fill cupcake liners 1/3 of the way up. Place 2-3 mini marshmallows over batter and continue to pour remaining batter no more than ¾ full.
Bake 15-18 minutes, until a cake tester or tooth-pick inserted into the center comes out clean. Remove and let cool completely.
Place marshmallows on a sheet plan and place under broiler and toast until golden brown.
Sprinkle tops of cupcakes with graham cracker crumbs and toasted mini-marshmallows. Enjoy!