Preheat your oven to 350°F (175°C). Line the muffin tin with cupcake liners.
In a large mixing bowl, whisk together the flour, granulated sugar, cocoa powder, baking soda, baking powder, and salt until thoroughly combined.
In another bowl, mix together the buttermilk, vegetable oil, eggs, vanilla extract, vinegar, and black food coloring until well blended.
Gradually add the wet ingredients to the dry ingredients, stirring gently until combined. Do not overmix.
Fill each cupcake liner about two-thirds full with batter.
Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center comes out clean. Let the cupcakes cool in the tin for 5 minutes, then transfer them to a cooling rack to cool completely.
While the cupcakes are cooling, prepare the marshmallow frosting. In a mixing bowl, beat the softened butter with an electric mixer until creamy.
Gradually add the powdered sugar and mix well. Then, mix in the heavy cream and vanilla extract until smooth.
Fold in the marshmallow fluff until fully incorporated.
Once the cupcakes have cooled completely, use a piping bag to swirl the marshmallow frosting onto each cupcake.
Optionally, you can sprinkle some cocoa powder or chocolate shavings on top for an elegant finish.