Preheat the oven to 350 degrees Fahrenheit. Line 2 muffin tins with 14 paper cupcake liners.
Combine the flour, sugar, cocoa powder, hot chocolate mix, baking powder, baking soda and salt in a medium bowl.
Combine the milk, oil, egg and vanilla in another bowl.
Slowly add the wet ingredients into the dry until just combined. Do not overmix as you will still mix in the coffee.
Add the hot coffee and stir to combine.
Divide the batter between the 14 cupcake liners. The batter is fairly thin so you can pour directly from the bowl into the liners and use an ice cream scoop or cookie scoop to catch the drips from the bowl as you fill each liner.
Bake for 16-18 minutes until a cake tester/toothpick inserted into the middle of a cupcake comes out clean or with a few moist crumbs.
Let the cupcakes cool in the pan for about 5 minutes and then transfer to a wire rack to cool completely.
Frost each cupcake with the marshmallow frosting using a piping bag or a knife. Sprinkle each cupcake with a generous amount of mini marshmallows or dehydrated marshmallows.