In a large bowl, cream together the butter, caster sugar and dark muscovado sugar until light in colour.
Crack the egg into the bowl and beat until combined. Add the flour and mix until the dough comes together.
Use your hands to work the dough into a ball, making sure you include any bits from around the edge of the bowl. Divide this ball into two equal sized balls. Wrap each in cling film and put in the fridge for an hour to firm up.
Preheat the oven to 180 degrees C. Prepare two baking trays by lining them with baking paper.
Remove your dough from the fridge. Dust your work surface and rolling pin with flour. Unwrap one ball and roll until approximately 3mm thick.
Use your ghost cookie cutter to cut 8 ghost shapes (you may need to reroll your dough scraps to get all the biscuits) and place them on one of the baking trays.
Do the same with the other dough ball. When these 8 biscuits are on their tray, use a skewer to make two eye holes in each ghost.
Put in the oven for approximately 10 minutes until firm - keep an eye on them because they can burn easily.
Remove from the oven and allow to cool on a wire rack.
Dust the ghost biscuits with eye holes with icing sugar. Spread the other ghosts with jam. Place the ghosts with eye holes on top, like a sandwich. Insert a silver ball into each eye hole, then serve.