Preheat your oven to 350°F (175°C) and line a muffin tin with cupcake liners.

In a large bowl, whisk together the flour, cocoa powder, baking powder, baking soda, and salt.

In a separate bowl, beat together the sugar and eggs until pale and fluffy; then mix in the milk, oil, and vanilla.

Gradually add hot water to the wet mixture, mixing gently until smooth.

Fold the dry ingredients into the wet mixture until just combined.

Fill each cupcake liner about two-thirds full with the batter.

Bake for 18-20 minutes, checking doneness with a toothpick.

Cool in the tin for 5 minutes before transferring to a wire rack.

Whip the heavy cream until soft peaks form; gradually add powdered sugar and cocoa until stiff peaks are achieved.

Frost cooled cupcakes and top with mini marshmallows.