1. Preheat the oven to 180°c, prepare a rectangle cake pan (approx 24cm x 13cm), grease with cooking spray and line with baking paper.
2. Combine 1 cup of water, sugar, 125g butter, cocoa and baking soda in a saucepan and warm, very low heat and then bring up to medium heat, stirring until the sugar has dissolved. Then bring to the boil and simmer gently for 2 minutes. Transfer mixture to a bowl and cool for 30 minutes.
3. Meanwhile, to prepare the decorations: melt two-thirds of the white chocolate melts in a bowl over simmering water or microwave, refer to packet instructions. Cool slightly and then spoon chocolate into a piping bag. Line a baking tray with greaseproof baking paper, place 3 biscuits down to create your headstones and cover the top side of the biscuit with white chocolate, or outline the edge as shown in the picture. Then directly on the baking paper, make some thin ghost shapes also using the white chocolate, and allow to set at room temperature. Refrigerate for 5 minutes.
4. Once hardened, remove your headstones from the refrigerator, use the black writing icing pen to write words and dates etc onto the headstones and ghost faces onto white chocolate. Refrigerate until required.
5. Once the cake mixture has cooled, stir in the flour and eggs, and beat until just combined. Pour into cake pan and bake for 40 minutes or until cooked. Cool for 10 minutes before turning onto a wire rack to cool completely.
5. Cream 125g unsalted butter until light and fluffy. Add the sifted icing sugar, milk and a few drops of green food colouring and beat again until combined. Cut a shallow "open grave" out of the centre of the top of the cake, then spread the green buttercream icing over the entire cake, heaping around the edges of the grave. Sprinkle liberally with the grated chocolate to resemble fresh dirt.
6. Remove your headstones from the refrigerator, and position them on the top of the icing, pushing them in slightly to hold. Place the white chocolate ghost faces around the headstones.
7. Line the outside of the cake with cadbury milk finger biscuits.
8. Place 4 licorice sticks in each of the corners of the cake. Then stretch out the rope licorice to make a fence around the grave and twirl it around the 4 corner licorice stands.