Preheat oven to 180°C. Remove butter for the icing from the fridge to come to room temperature. Line a muffin tray with cupcake liners. Bring a kettle to the boil.
Place butter and cocoa mix in a medium bowl and pour over boiling water measure. Whisk until combined. In a second large bowl, combine cake mix and salt measure. Pour wet mix into dry mix and whisk to combine. Add egg and milk and fold to combine.
The cupcake batter is very runny, don't worry, it's meant to be! Evenly fill the cupcake liners with batter, about 3/4 full. Bake for about 20 minutes, until a skewer inserted comes out clean.
Place butter in a large bowl or bowl of your stand mixer. Beat for about 5 minutes, until pale. Add icing sugar and milk and continue to beat on low speed for 10 minutes (yes, 10 minutes!) so the icing will be super fluffy.
Wait until cupcakes are completely cooled before icing or the icing will melt. Place icing into a piping bag or snaplock bag and cut a hole (about 2cm wide). Hold the bag directly above the cupcake and pipe blobs onto the cake, lifting up quickly at the top to create a ghost shape. Pop three chocolate chips onto the icing, two for the eyes and one for the mouth.