Preheat your oven to 350°F (175°C). Line a cupcake tin with cupcake liners.
In a large mixing bowl, combine the flour, sugar, cocoa powder, baking soda, baking powder, and salt. Whisk until well blended.
In another bowl, whisk together the buttermilk, vegetable oil, eggs, vanilla extract, black food coloring, and vinegar until smooth.
Gradually add the wet ingredients to the dry ingredients, mixing until just combined. Do not overmix.
Fill each cupcake liner about two-thirds full with the batter. Bake in the preheated oven for 20-25 minutes, or until a toothpick inserted in the center comes out clean.
Allow the cupcakes to cool in the tin for 5 minutes, then transfer them to a cooling rack to cool completely.
For the marshmallow frosting, beat the softened butter in a mixing bowl until creamy. Gradually add the powdered sugar, mixing well.
Add the marshmallow fluff, vanilla extract, and pinch of salt. Continue to beat until the frosting is fluffy and light.
Once the cupcakes are completely cool, frost them generously with the marshmallow frosting using a piping bag or a spatula.
If desired, top with additional marshmallows for decoration.