Preheat your oven to 350°F (175°C). Line the cupcake pan with cupcake liners.
In a large mixing bowl, sift together the flour, granulated sugar, cocoa powder, baking soda, and salt until well combined.
In another mixing bowl, whisk together the buttermilk, vegetable oil, eggs, vanilla extract, and white vinegar until smooth.
Gradually add the wet ingredients to the dry ingredients, mixing gently until just combined. Be careful not to overmix.
Divide the batter evenly among the cupcake liners, filling each about two-thirds full.
Add about 3-4 mini marshmallows into the center of each cupcake batter before baking.
Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted in the center comes out clean.
Let them cool in the pan for 5 minutes, then transfer to a cooling rack to cool completely.
Once cooled, use a piping bag or a spatula to generously top each cupcake with marshmallow fluff.
Optionally, garnish with chocolate shavings or a sprinkle of cocoa powder for an extra touch.