Preheat oven to 350 degrees. Line muffin pans with baking cup liners. This recipe will make 24 cupcakes. Place oven racks in the top and bottom third of oven. You will be swapping rack positions and rotating pans halfway through.
In the bowl of a stand mixer fitted with the paddle attachment, stir together sugar, flour, cocoa, baking soda, baking powder and salt. Add buttermilk, warm coffee, oil and vanilla. Mix.
Add eggs, one at a time, and beat on medium speed until fully mixed in. Batter will be thin. Fill muffin cups about ⅔ full. I use a medium (¼ cup) ice cream scoop.
Bake at 350 degrees for 20-25 minutes, rotating your pans halfway through (just give them a 180˚ spin) and swapping the rack positions of the pans. They are done when a toothpick or skewer inserted in the center comes out almost completely clean. Cool the cupcakes in the pan for 5 minutes, then remove from the pan and let cool completely on a cooling rack.
When your cupcakes are fully cooled, about an hour or so, use a small ring mold or an apple corer to remove the center of your cupcake. This is where you will pipe in the marshmallow filling.