Make the cupcakes:

1. Preheat the oven to 350°F (175°C). Line a standard muffin tin with paper liners.
2. Whisk together the flour, sugar, cocoa powder, baking soda, baking powder, and salt in a large bowl.
3. In a separate bowl, whisk together the eggs, buttermilk, oil, and vanilla. Pour the wet ingredients into the dry and stir to combine.
4. Stir in the hot coffee. The batter will be thin — this is normal.
5. Fill the lined muffin cups about 2/3 full. Bake for 18–22 minutes, until a toothpick inserted in the center comes out clean. Cool in the pan for 5 minutes, then transfer to a wire rack and cool completely before frosting.

Make the frosting:

1. In a large saucepan over medium heat, whisk together the butter, sugar, marshmallows, and water.
2. Bring to a boil, stirring constantly to prevent burning.
3. Once boiling, remove from the heat immediately.
4. Whisk in the chocolate chips until fully melted and smooth.
5. Allow to cool completely. Once cooled, whisk until lighter and fluffier.
6. Spread the frosting onto the cooled cupcakes. For best results, refrigerate until firm, or serve at room temperature.