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

In a mixing bowl, whisk together flour, sugar, cocoa powder, baking powder, baking soda, and salt.

In another bowl, mix the egg, milk, vegetable oil, and vanilla extract.

Combine the wet ingredients with the dry ingredients and mix until just combined.

Pour the batter into the prepared muffin tin, filling each liner about 2/3 full.

Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean.

Let the cupcakes cool completely on a wire rack.

In a saucepan, combine mini marshmallows and water over low heat, stirring until melted.

Remove from heat and gradually stir in powdered sugar until a thick mixture forms.

Allow the mixture to cool slightly, then use a piping bag to pipe ghost shapes on top of each cupcake.

Melt dark chocolate chips and use a toothpick to dot eyes on the marshmallow ghosts.