Prepare the chocolate cake according to your favourite recipe. Allow to cool fully.
Cut the marzipan into three equal pieces. Add varying amounts of green food colouring to each piece to achieve different shades of green. Knead each piece separately to get an even colour throughout.
Thinly roll out each piece of marzipan on a work surface dusted with icing sugar. Cut into equal-sized squares.
In a shallow dish, combine about 50g of icing sugar with water. Add enough water so that the mixture is very thin, about the consistency of cream.
Dip the back of a square in the icing sugar 'glue', then place on top of the cake. Repeat with remaining squares, alternating between the three shades of green in a random fashion, working from the outer edges of the cake towards the centre. In the centre, leave two squares missing for the Creeper's eyes. Then, just below the eyes, create a mouth (you will need to cut some of the squares to get the mouth just right).
I used this recipe for a chocolate hazelnut cake, which makes the exact size needed here. You can use any chocolate cake for this recipe, however!