In a medium sized mixing bowl, beat together the butter and salt for 2-3 minutes, until light and fluffy.
Slowly beat in the powdered sugar until all of it has been used.
Add in the marshmallow fluff, milk, and vanilla extract. Beat for an additional 2-3 minutes, until the mixture is light and fluffy.
Push down in the center of the cupcakes to make a divot, or use a large round piping tip to scoop out the center of the cupcakes.
Fit a piping bag with a round tip and fill it with marshmallow fluff, then pipe some marshmallow fluff into each of the cupcakes.
Transfer the marshmallow frosting to a piping bag fitted with a large round tip, and generously frost each cupcake.
In a small bowl, whisk together the black cocoa powder and water until well combined. Use a food-safe paint brush to paint little ghost faces on to each of the cupcakes, then serve and enjoy!