Sift the icing sugar into a large bowl. Add 5 teaspoons of water and mix until you achieve a thick but pourable consistency.

Prepare your workspace by laying out the cookies on a sheet of baking paper or a wooden board.

Dip the marshmallows: One by one, dip the bottoms into the icing. Place each marshmallow at the center of a cookie.

Using a spoon, carefully pour the white icing over each marshmallow, allowing it to drizzle down the sides of the cookie for a spooky effect.

Allow the cookies to set, then use the black writing icing to create eyes and a mouth on each marshmallow ghost.