Preheat oven to 350°F (180°C). Spray donut pan with non-stick spray. Set aside.
Make the donuts: Whisk the flour, cocoa powder, baking powder, baking soda, nutmeg, espresso powder, and salt together in a large bowl. Set aside.
120 g all-purpose flour, 20 g cocoa powder, 1 tsp baking powder, ¼ tsp baking soda, ½ tsp ground nutmeg, ¼ tsp espresso powder, ½ tsp coarse sea salt
Whisk the melted butter, buttermilk, vanilla, egg, and sugars together. Pour into the dry ingredients and whisk everything together there are no pockets of flour; be careful not to overmix (otherwise you’ll have tough donuts.) Batter will be slightly thick and there will be lumps.
30 g unsalted butter, 120 g buttermilk, 1 tsp vanilla extract, 1 large egg, 50 g granulated sugar, 55 g dark brown sugar
Pipe the batter into each cavity in the prepared donut pans. They should only be about 2/3 full - if you fill them too much you'll lose the hole in the center of the donut.
Bake for 8-10 minutes. To test, poke your finger into the top of the donut. If the donut bounces back, they’re done. Cool donuts for 2 minutes then transfer to a wire rack; let them cool completely before adding the ganache. Re-grease the pan and bake the remaining donut batter.
Make the ganache: Make the ganache while the donuts are cooling. Add the chocolate to a bowl and set aside. Add the heavy cream to a medium saucepan over medium heat on the stovetop. Heat the cream, stirring occasionally, until it starts to boil and bubble up. Give it a stir and pour into the bowl of chocolate. Let sit for 30 seconds, then whisk until smooth. Once smooth, whisk in the butter until melted and combined.
120 g heavy cream, 115 g dark chocolate, 15 g unsalted butter
Assemble: Dip each donut into the ganache and set (ganache-side up) on a cooling rack that has a piece of wax paper underneath (to catch the drips). Immediately sprinkle with the graham cracker crumbs and place a few mini marshmallows on top of each donut.
crushed graham crackers, mini marshmallows
Donuts are best served immediately. Leftovers keep well covered tightly at room temperature for up to 2 days.