To make the ghosts, heat oven to 110C/90C fan/gas ¼. Whip the egg white in a clean bowl until stiff peaks form. Whisk in the sugar a tbsp at a time and keep whisking for a couple of mins until the mixture is thick and resembles shaving foam. Gently spoon the mixture into a large freezer bag, then cut a 1.5cm hole in one of the corners. Cover a baking sheet with some baking parchment. Carefully squeeze a small circle of whipped egg white out of the bag, pulling upwards as you do to make a ghost shape. Repeat until the mixture is used up - you should get about 15 ghosts. Bake for 1½ hrs until crisp. Can be stored in an airtight container for up to 2 days.
Now make the cake. Heat oven to 180C/160C fan/gas 4. Tip the cocoa powder, self-raising flour and sugar into a large bowl, breaking up any clumps of sugar. Mix together the eggs, milk and oil in a measuring cup or bowl, then pour over the dry ingredients and stir everything together until smooth. Grease and line a deep baking dish (20 x 30 x 5cm) with baking parchment. Pour in the cake mixture and bake for 30 mins. Leave to cool, then turn out onto a serving plate. Alternatively, wrap well and store for up to 2 days.
Finish decorating the cake: heat cream in a saucepan until just boiling. Place the dark chocolate in a large bowl and pour over the hot cream. Stir until the chocolate melts. Use a clean brush to paint a layer of chocolate over 7 rich tea finger biscuits, then set aside to cool. Pour the rest of the chocolate mixture over the cake and smooth over with a knife. Whizz the chocolate cookies, or bash in a freezer bag with a rolling pin, until small crumbs form. Sprinkle over the top of the cake.
Place the white chocolate in a small bowl, set over a pan of simmering water. Leave for 5 mins or until melted, then spoon into a small freezer bag. Wait for 10 mins so the mixture is not too runny, then cut a tiny hole in one corner of the bag. Pipe out 2 small blobs onto each ghost, place a silver ball on each to make eyes, then pipe out suitable words and shapes on the gravestones. Leave for 30 mins to set, then push the biscuit gravestones into the cake and arrange the ghosts around. To get the ghosts to a€˜fly’, push a thin wire (you can get these from a florist shop - remember to remove before eating) into the bottom of the ghost, then place in the cake, hiding the wire behind a gravestone.