Preheat the oven to 180°C (160°C for fan assisted ovens)/Gas Mark 4. Grease and line 2 x 18cm deep round cake tins with baking paper.
Make up the cake mix with the vegetable oil, water and eggs according to the pack instructions. Divide equally between the cake tins and level the tops. Bake for 30-35 minutes or until a round bladed knife inserted into the centre of the cakes comes out clean. Leave in the cake tins for 10 minutes. Remove from the tins and place on a wire rack to cool. Peel off the baking paper. Cool completely; approximately 1 hour.
Halve each cake horizontally so you have 4 equal layers and sandwich the layers together using three-quarters of 1 tub of icing. Place the cake onto a cake board the same size diameter as the cake. Spread three-quarters of the second tub of icing over the top and sides of the cake to form a nice smooth layer. Place into the fridge to chill for 30 minutes.
Place the chocolate in a heatproof bowl. Heat the cream in a small pan until just boiling. Remove from the heat and pour over the chocolate. Let the chocolate melt for 2 minutes, then stir until completely smooth.
Place the cake onto a wire rack over a baking tray. Pour the chocolate ganache over the top of the cake and using a palette knife help the ganache spread down the sides of the cake. Neaten with the palette knife and leave to set for 30 minutes.
Spoon the remaining vanilla icing into a piping bag fitted with a 1.5cm round nozzle. Pipe about 20 individual ghosts onto the top of the cake starting in the middle and placing a pair of candy eyes on each as you go. Press 12 edible ‘eyes’ into the sides of the cake for a super spooky Halloween ghost cake!