Preheat the oven to 180 degrees/gas mark 4. Cream together the butter and sugar in a mixing bowl. Add the eggs and the flour gradually, folding as you go.
Divide the mixture into two bowls, so that you can make 6 double chocolate cupcakes (add the cocoa powder, dark chocolate chips and milk), and 6 vanilla cupcakes (add the vanilla extract).
Prepare 12 cupcake cases in a tin. Gradually add the mixture until you have filled the cases three quarters of the way up. Try to fill each case up with the same amount of mixture so that they have a uniform rise for decorating.
Place them in the oven for 15-20 minutes, or until golden brown.
Place the cupcakes on a wire rack and leave to cool.
To decorate, you will need green, black and white icing. Begin by making the orange for the pumpkin cupcakes. Gradually add drops of orange food dye to a small ball of white icing and knead until the icing is a bold orange in colour. Roll out, and using a circular cookie cutter, remove a small circle the size of the top of the cupcake. Place this on top.
Repeat this with the remaining cupcakes. You will need 6 white circles (3 for Jack Skellington faces, 3 for mummy faces), 3 orange circles (pumpkins) and 3 green circles (frankenstein or witch faces). Using the black icing, decorate each of the faces carefully and place the features onto the cupcakes (you may need to use water as an adhesive). Serve once complete.