Make the Cupcakes
Preheat oven to 350°F (175°C) and line a muffin pan with cupcake liners.
In a bowl, whisk together flour, cocoa powder, baking soda, baking powder, and salt.
In another bowl, mix granulated sugar, brown sugar, oil, eggs, vanilla, and milk until smooth.
Gradually add the dry ingredients to the wet ingredients.
Stir in hot water or coffee until the batter is smooth (batter will be thin).
Divide batter evenly among liners, filling about ⅔ full.
Bake for 18–20 minutes, or until a toothpick inserted comes out clean.
Cool completely before frosting.
Make the Frosting
Beat butter until creamy.
Add powdered sugar and cocoa powder gradually.
Mix in vanilla and cream until smooth and fluffy.
Assemble the Cupcakes
Frost cooled cupcakes with chocolate frosting.
Top with marshmallow fluff or place marshmallow halves on top.
Optional: lightly toast marshmallows with a kitchen torch.