Preheat oven to 350°F. Grease two doughnut pans with cooking spray. If you only have one, you'll have to bake the doughnuts in batches. Set to the side.

To make the doughnuts: In a large bowl, combine the flour, brown sugar, cocoa powder, baking powder, baking soda, salt, espresso powder and cinnamon. Make a well in the center of the bowl. In a separate bowl, whisk together the eggs, buttermilk, vanilla extract, and melted butter. Pour the wet ingredients into the dry and fold until they're just combined. Do not over-mix.

Fill the prepared doughnut pan about ¾ of the way up. Bake for about 10 to 12 minutes or until a toothpick inserted in the middle comes out clean. Remove from the oven and allow to cool down in the pan. Transfer the doughnuts to a wire rack and allow to cool completely. If you only have one pan, keep baking the doughnuts in batch until all the batter is used. Allow to cool completely before glazing.

To make the glaze: Combine the butter and 1/4 cup water in a small saucepan, and cook over medium until the butter has melted. Add the mini marshmallows and cook, stirring often, until they've completely melted. Transfer to a large bowl, and whisk together with the sugar, salt and vanilla until well combined. Let cool slightly.

Dip the doughnuts, one at a time, allowing the excess to drain off. Place on a wire rack set over a baking sheet. While the glaze is still well, sprinkle with graham cracker crumbs and a large marshmallow. Let sit for about 5 to 10 minutes to set. Toast the marshmallows with a kitchen torch until charred. Garnish with a piece of chocolate. Serve immediately or wrap in plastic and store at room temperature for up to 2 days.