Pipe the buttercream onto each cupcake- you are aiming for a tall-ish cone in the centre of the cupcake. To achieve this, hold the piping bag vertically in the centre of the cake, then apply gentle pressure, lifting as you go to add height. Place the iced cakes into the fridge for about 15 minutes, to allow the buttercream to firm up, while you make the other decorations.
To make the chocolate fedoras, warm a small frying pan over medium-high heat. Once the pan is hot, turn off the heat. Use the heat from the pan to melt the base of a Rolo (be careful of your fingers near the hot pan!). When the base has melted a little, stick the Rolo to the flat side of a giant chocolate button, making sure it is centred. Repeat 11 more times, for a total of 12 chocolate fedoras, then set all the chocolates aside to harden.
When the buttercream on the cakes is firm, you can start making the 'sheets' for the ghosts. Knead your modelling chocolate to bring it back to a smooth, pliant dough, then break off a small piece (about 15g). Dust your work surface and rolling pin with icing sugar, then roll out the ball, keeping the chocolate in a roughly circular shape, very thinly (about 12cm diameter). Use a small, sharp knife to cut off any messy edges with a wavy line, then carefully lift up the chocolate and drape it over the chilled cupcake. Press it against the central buttercream dollop and arrange the folds evenly.
Before the modelling chocolate firms up, press two dark chocolate chips into the chocolate, to act as eyes. Using the black edible marker, draw a bar connecting the two chips to create the sunglasses (see Note 3).
Heat the same, small frying pan you used to make the fedoras back up over a medium heat. Once the pan is hot, turn off the heat and use the warmth of the pan to melt the bottom of each fedora slightly (this will allow them to stick to the top of the modelling chocolate). Press one fedora on top of each ghost cupcake. Be careful not to over-melt the chocolate, or it will ooze out when you push it down onto the cupcake.
I recommend adding the chocolate decorations (from the modelling chocolate 'sheets' onwards) on the day you plan to serve the cakes, to prevent any 'sweating'. The iced cupcakes will keep for 2-3 days in an airtight container (or the fridge), while the fully decorated cupcakes are best eaten on the same day.