Prepare the cakes: Preheat oven to 375°F (190°C). Line baking sheets with parchment paper.
Cream butter and sugar: In a large bowl, cream together the softened butter and granulated sugar until light and fluffy.
Add eggs and vanilla: Beat in the eggs one at a time, then stir in the vanilla extract.
Combine dry ingredients: In a separate bowl, whisk together the flour, cocoa powder, baking soda, and salt.
Combine wet and dry ingredients: Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk, beginning and ending with the dry ingredients. Mix until just combined.
Add coffee: Stir in the hot coffee (or hot water). The batter will be thin.
Bake: Drop rounded tablespoons of batter onto the prepared baking sheets, leaving about 2 inches between cookies. Bake for 10-12 minutes, or until the tops spring back lightly when touched.
Cool: Let the cookies cool on the baking sheets for a few minutes before transferring them to a wire rack to cool completely.
Make the filling: While the cakes are cooling, prepare the filling. In a large bowl, cream together the softened butter and powdered sugar until light and fluffy. Stir in the marshmallow cream and vanilla extract. Add milk or cream, 1 tablespoon at a time, if needed, to achieve the desired spreading consistency.
Assemble the whoopie pies: Once the cakes are completely cooled, spread a generous amount of filling on the flat side of one cake and top with another cake. Gently press together to form a whoopie pie.
Serve: Serve immediately or store in an airtight container.